Fancy wife joining that man (4)
I believe the answer is:
whim
'fancy' is the definition.
(I've seen this before)
'wife joining that man' is the wordplay.
'wife' becomes 'w'.
'joining' is a charade indicator (letters next to each other).
'that man' becomes 'him' (I've seen this before**).
'w'+'him'='WHIM'
(Other definitions for whim that I've seen before include "Fanciful idea" , "Sudden or capricious idea" , "'Caprice, notion (4)'" , "Passing fancy" , "Fancy, notion" .)