Limited sex during punishment (6)
I believe the answer is:
finite
'limited' is the definition.
(similar in meaning)
'sex during punishment' is the wordplay.
'sex' becomes 'it' (a common euphemism).
'during' means one lot of letters goes inside another.
'punishment' becomes 'fine' (fine is a kind of punishment).
'it' inserted into 'fine' is 'FINITE'.
(Other definitions for finite that I've seen before include "Having limit, not endless" , "within bounds" , "Having boundaries, limited" , "Having a limit, not completely open-ended" , "Of limited size" .)