Fancy wife joining that fellow (4)
I believe the answer is:
whim
'fancy' is the definition.
(both can mean a notion or desire)
'wife joining that fellow' is the wordplay.
'wife' becomes 'w' (abbreviation).
'joining' is a charade indicator (letters next to each other).
'that fellow' becomes 'him' ('him' refers to a man).
'w'+'him'='WHIM'
(Other definitions for whim that I've seen before include "Sudden impulse" , "Sudden odd desire" , "Sudden unusual change of mind" , "Sudden or capricious idea" , "Fad" .)