Wife leaving weakling for trouble-maker (3)
I believe the answer is:
imp
'trouble-maker' is the definition.
'wife leaving weakling' is the wordplay.
'wife' becomes 'w' (abbreviation).
'leaving' suggests deleting specific letters.
'weakling' becomes 'wimp' (wimp is a kind of weakling).
'wimp' with 'w' removed is 'IMP'.
'for' acts as a link.
(Other definitions for imp that I've seen before include "Small mischievous sprite" , "Naughty child" , "Little mischief-maker" , "Mischievous monkey" , "Naughty demon" .)