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I don't know if the word "clerk" is very widespread in your country (I sometimes see "office workers" being used instead but in Malaysia if you ask people what type of jobs they hold you'll often be told Saya kerani (I'm a clerk) so learn the word kerani well (at least know what it means when you hear it). Does that word ring a bell? Do you think you have come across that word already? In that case the chances are that you're confusing it with the word kerana which means "because" and which you have already learnt in Lesson 25.
While we're at it I guess there's no harm bringing up two more words quite similar so you won't ever mix them up. One is kereta meaning "a car" and the other kelabu meaning "grey" in colour.
And in case you have an enemy and want to make it known you might need to learn this sentence!
Dia bukan kawan saya. Dia musuh saya. (He is not my friend. He is my enemy).
There is a second word for enemy (starting with s*) but I don't think you have that many enemies to need to learn another word for it!

Further examples with siapa:
(When answering the telephone) Helo, Peter di sini. Siapa itu? (Hello, Peter here. Who's calling?)
Siapa orang lelaki itu? - Dia guru bahasa saya. (Who is that man? - He is my language teacher.)
Siapa orang perempuan itu? - Dia isteri saya. (Who is that woman? - She is my wife.)
In all the above examples I have used Siapa at the beginning of the sentence. However it can also appear at the end, as in the following sentences:
Anda cari siapa? (Who are you looking for?)
Saya cari kawan saya. (I'm looking for my friend.)
cari or mencari means to search or to look for someone or something eg.
Dia sedang mencari kunci keretanya. (=He is searching for his car keys.)
Incidentally (as you will learn in Lesson 49) the pronunciation of the syllable "ca" in cari is not ka but cha as in cha-cha-cha so the word is pronounced as cha-ri.
Ini untuk siapa? (Who is this for?)
Ini untuk bapa saya. (This is for my father.)

  Family Relationships  

I think this is a good place to bring in the Malay words for the most important members of the family. Here is the core vocabulary for family members (some of which you have already come across but I'm adding them here to make the list complete)
One thing to note though. Apart from the linguistic aspect, there is also the cultural aspect to be kept in mind when you are talking about your siblings in Malay. This is because while in English we can just say "He is my brother" or "She is my sister" and leave it at that (that is, without revealing if he or she is older or younger than yourself), this is simply not possible in Malay where you have to specify each time if it is your younger or elder brother or sister you are talking about. The only possible case for ambiguity is in the case of a younger sibling where you can leave out the gender eg. Adik saya tinggal di New York will leave the listener in doubt as to whether you are talking about a younger brother or a younger sister. (To make it clear you will have to say adik lelaki for a younger brother or adik perempuan for a younger sister). This will not be necessary though, if the person is with you, as the situation will show if it's your younger brother or sister so in such a case you can just say Ini adik saya.
Another thing to note is that just as Malaysians are fond of using the word "uncle" or "auntie" when addressing a man or woman who is older than you (out of politeness), adik can also be used to address a child, even if he is unrelated to you. A woman could also address her husband lovingly as abang. In such a case it doesn't mean that he is her elder brother at all! So let's look at the list of the most common family members below:

father = bapa or ayah
mother = emak or ibu
daughter = anak perempuan
son = anak lelaki
brother (younger) = adik lelaki
brother (elder) = abang
sister (younger) = adik perempuan
sister (elder) = kakak (Note that in Indonesia kakak can mean not only "elder sister" but also "elder brother").
husband = suami
wife = isteri
aunt = emak saudara (spoken form: mak cik)
uncle = bapa saudara (spoken form: pak cik)
grandfather = datuk**
grandmother = nenek
grandson = cucu lelaki
granddaughter = cucu perempuan
nephew = anak saudara lelaki
niece = anak saudara perempuan
cousin = saudara sepupu
father-in-law = bapa mertua
mother-in-law = ibu mertua
son-in-law = menantu lelaki
daughter-in-law = menantu perempuan
brother-in-law (older than you) = abang ipar
brother-in-law (younger) = adik ipar lelaki
sister-in-law (older) = kakak ipar
sister-in-law (younger) = adik ipar perempuan
step-mother = ibu tiri
step-father = bapa tiri
adopted child = anak angkat

Quite a handful eh? The best way to study this category of vocabulary is to write the names of your family members on a piece of paper and then their relationships to you eg.
Tom adalah adik lelaki saya (replace "adik lelaki" with abang or bapa saudara or suami, etc.)
Mary adalah adik perempuan saya (replace "adik perempuan" with kakak or emak saudara or isteri, etc.)
So whenever you are asked Siapa dia? (= Who is he/she?) you will have your answer ready.
Besides this is a much more meaningful way of learning such words than if you were to learn them by rote off a list.

* The word is seteru.
**Please note that Datuk (with a capital letter) is a title awarded to prominent Malaysians by the Sultans or King on their birthdays. It is usually followed by the person's name though you normally use the word by itself without the name when you are addressing the person directly. But if you are a nondescript like me, you can still "earn" that title by becoming a grandfather!