Make fun of wife's parting boob (4)
I believe the answer is:
twit
'make fun of' is the definition.
(thesaurus)
'wife's parting boob' is the wordplay.
'wife' becomes 'w'.
'parting' is an insertion indicator.
'boob' becomes 'tit' ('tit' can be a synonym of 'boob').
'w' going into 'tit' is 'TWIT'.
(Other definitions for twit that I've seen before include "Silly person; tease" , "Ninny" , "Slangily a foolish person" , "Twerp" , "Silly person (colloq.)" .)