Perpetrator of a wrongdoing (7)
I believe the answer is:
culprit
'perpetrator of a wrongdoing' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for culprit that I've seen before include "baddie" , "he should have conviction" , "The one responsible for a crime" , "one awaiting trial" , "Criminal" .)