Retaliate on behalf of (6)
I believe the answer is:
avenge
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">retaliate on behalf of</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 avenge that I've seen before include "Inflict retribution" , "Take satisfaction for injury or wrong" , "Exact retribution, take satisfaction for injury" , "Requite, repay" , "Indulge in retaliation" .)