Nerve gas produced by agents of retribution (8)
I believe the answer is:
avengers
'agents of retribution' is the definition.
(I've seen this before)
'nerve gas produced' is the wordplay.
'produced' indicates an anagram.
'nerve'+'gas'='nervegas'
'nervegas' is an anagram of 'AVENGERS'.
'by' acts as a link.
(Other definitions for avengers that I've seen before include "Agents of retribution" , "They seek or inflict retribution" , "1960s TV series with Steed and Emma Peel" , "Retaliating fighters" , "People seeking retaliation" .)