Act of retaliation (9)
'act of retaliation' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for reprisal that I've seen before include "hitting back" , "compensation" , "Counterattack" , "Retaliatory action against an enemy" , "revenge attack" .)
(Other definitions for revenge that I've seen before include "Retribution" , "Malicious retaliation" , "An eye for an eye" , "Ship remembered in verse" , "Satisfaction taken for injury" .)
(Other definitions for vengeance that I've seen before include "Retribution" , "Retributory punishment" .)
(Other definitions for tit for tat that I've seen before include "You hit me, I hit you" , "In annoyance, return" , "An eye for an eye" , "Repayment in kind" , "Retaliation in kind" .)