One exacting retribution (7)
I believe the answer is:
avenger
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">one exacting retribution</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)