Fit father's engaged in kinky sex? (5)
I believe the answer is:
spasm
'fit' is the definition.
(thesaurus)
'father's engaged in kinky sex?' is the wordplay.
'father's' becomes 'pas' (pa is a kind of father).
'engaged in' is an insertion indicator.
'kinky sex?' becomes 'sm' (I've seen this in other clues).
'pas' put into 'sm' is 'SPASM'.
(Other definitions for spasm that I've seen before include "Muscle twitch" , "Painful muscle contraction" , "Tic, sudden contraction" , "Fit" , "Sudden convulsion" .)