An act of retaliation (8)
I believe the answer is:
reprisal
'an act of retaliation' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for reprisal that I've seen before include "Retaliatory action against an enemy" , "comeback" , "Retaliatory deed" , "Counterattack" , "Tit for tat" .)