Plonker, partner coming after wife (5)
I believe the answer is:
wally
'plonker' is the definition.
(I know that plonker can be written as wally)
'partner coming after wife' is the wordplay.
'partner' becomes 'ally' (an ally is an associate or supporter).
'coming after' means one lot of letters go next to another.
'wife' becomes 'w' (abbreviation).
'ally' after 'w' is 'WALLY'.
(Other definitions for wally that I've seen before include "Idiot (slang)" , "A fool (sl.)" , "Complete idiot" .)