Agent rails about retribution (8)
I believe the answer is:
reprisal
'retribution' is the definition.
(I know that reprisal is a type of retribution)
'agent rails about' is the wordplay.
'agent' becomes 'rep' (short for representative).
'about' is an anagram indicator.
'rails' anagrammed gives 'risal'.
'rep'+'risal'='REPRISAL'
(Other definitions for reprisal that I've seen before include "Retaliatory deed" , "revenge attack" , "compensation" , "hitting back" , "Tit for tat" .)