Where plumber has spare part, hopefully installing one for nothing (2,4)
I believe the answer is:
in vain
'for nothing' is the definition.
(I've seen this in another clue)
'where plumber has spare part hopefully installing one' is the wordplay.
I cannot really see how this works, but
'part' could be 'in' (synonyms) and 'in' is present in the answer.
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
The remaining letters 'van' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for in vain that I've seen before include "With no profitable outcome, a waste of time" , "Unsuccessfully" , "Uselessly, to no avail" , "To no purpose" , "To no end" .)