This is rather a complicated question for me.
A plumber is a person --> what person?
attributive defining clause
whose job --> ???
that it is to repair pipes.
Nothing is clear, sorry.
I can not ask questions from main clauses to the subordinate ones.
Therefore I can not see any sense, I have to confess.
Maybe I am caught in an endless loop with this plumber.
Could you find another example of such elipsis and reveal it to me please?
Or explain from another point of view.