A sin? Certainly (6)
I believe the answer is:
agreed
'certainly' is the definition.
(both can be used to show agreement)
'a sin?' is the wordplay.
'sin?' becomes 'greed' (greed is a kind of sin).
'a'+'greed'='AGREED'
(Other definitions for agreed that I've seen before include "Done!" , "Consented and concurred" , "Said yes" , "Held the same opinion" , "OK, fine!" .)