A mortal sin admitted (6)
I believe the answer is:
agreed
'admitted' is the definition.
(I've seen this before)
'a mortal sin' is the wordplay.
'mortal sin' becomes 'greed' (greed is a kind of mortal sin).
'a'+'greed'='AGREED'
(Other definitions for agreed that I've seen before include "Okayed" , "Settled by common consent" , "ratified" , "In accord" , "Geared (anag.)" .)