A deadly sin done! (6)
I believe the answer is:
agreed
'done' is the definition.
(thesaurus)
'a deadly sin' is the wordplay.
'deadly sin' becomes 'greed' (greed is a kind of deadly sin).
'a'+'greed'='AGREED'
(Other definitions for agreed that I've seen before include "'Concurred, assented (6)'" , "Gave consent" , "Accepted; tallied" , "Was consistent" , "Saw eye to eye" .)