Chap admitting old complaint (4)
I believe the answer is:
moan
'complaint' is the definition.
(a moan is a complaint or objection)
'chap admitting old' is the wordplay.
'chap' becomes 'man' (chap can mean a man).
'admitting' means one lot of letters goes inside another (admit can mean to welcome in or accept).
'old' becomes 'o' (abbreviation).
'man' placed around 'o' is 'MOAN'.
(Other definitions for moan that I've seen before include "Indicate pain" , "Low sound of pain" , "Complaint" , "Keen" , "Express pain" .)