subject verb
A plumber is a person, /and her
job is to repair the pipes/.
A plumber is a person /whose
job is to repair the pipes/.
A plumber is a person, /and her
job it is to repair the pipes/.
Or should I place / between job and it? How many subordinate clauses here?
Don't you try to say that there are two?
I still can not catch the idea, and you will be tired of this question soon.
But! Is a plumber is usually a woman in your country?
That is pretty funny

.
Ok, let's stop joking.
We have two subjects, the first is a notional word "job" and the the second one is a substitute for "job". I have never heard about "head" as a part of a sentence. If you will send me some link to read, I will be grateful.
I can understand that
it is very often used as a formal subject in impersonal statements.
It is raining.
But I think it is not our case.
Why should I put a formal subject here if I have a real one (job)?
Michael