Talk again about environmentally damaging contributor to agricultural growth (6)
I believe the answer is:
repeat
'talk again' is the definition.
(say something again)
'about environmentally damaging contributor to agricultural growth' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'environmentally damaging contributor to agricultural growth' becomes 'peat' (peat can damage the environment).
're'+'peat'='REPEAT'
(Other definitions for repeat that I've seen before include "Echo words" , "Iterate" , "No new programme" , "Say, do or occur again" , "Reiterate" .)