One exacting retribution (7)
I believe the answer is:
avenger
'one exacting retribution' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for avenger that I've seen before include "He seeks satisfaction or retribution" , "Someone wanting to get even" , "One who retaliates -- engrave (anag)" , "Fury results" , "One getting his own back" .)