Tit for tat, say, not once backfired (7)
I believe the answer is:
revenge
'tit for tat' is the definition.
'say not once backfired' is the wordplay.
'say' becomes 'eg'.
'not once' becomes 'never' (I've seen this before).
'backfired' says the letters should be written in reverse.
'eg'+'never'='egnever'
'egnever' back-to-front is 'REVENGE'.
(Other definitions for revenge that I've seen before include "Malicious retaliation" , "An eye for an eye" , "Retribution" , "Satisfaction taken for injury" , "Ship remembered in verse" .)