An eye for an eye (9)
(Other definitions for vengeance that I've seen before include "Retributory punishment" , "Retribution" .)
'an eye for an eye' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tit for tat that I've seen before include "reprisal" , "satisfaction" , "revenge" , "protection given to Barnet" , "Retaliation called for" .)