Irritation starts to concern him after sex (4)
I believe the answer is:
itch
'irritation' is the definition.
(thesaurus)
'starts to concern him after sex' is the wordplay.
'starts to' indicates taking the first letters.
'after' means one lot of letters go next to another.
'sex' becomes 'it' (a common euphemism).
The first letters of 'concern him' is 'ch'.
'ch' put after 'it' is 'ITCH'.
(Other definitions for itch that I've seen before include "Feel tingling sensation in skin" , "Burning" , "burn" , "that's irritating" , "Hankering" .)