One about to punch sick sex criminal (7)
I believe the answer is:
illicit
'criminal' is the definition.
(I've seen this before)
'one about to punch sick sex' is the wordplay.
'one' becomes 'i' (Roman numeral).
'about' becomes 'c' (abbreviation for circa).
'to punch' indicates putting letters inside (some letters punch a hole in others).
'sick' becomes 'ill' (similar in meaning).
'sex' becomes 'it' (a common euphemism).
'i'+'c'='ic'
'ill'+'it'='illit'
'ic' going within 'illit' is 'ILLICIT'.
(Other definitions for illicit that I've seen before include "Prohibited" , "Not allowed, unlawful" , "Contraband" , "Contrary to morality or convention" , "Felonious" .)