Green politician in party starts to spout environmental rot (9)
I believe the answer is:
decompose
'rot' is the definition.
('decompose' can be a synonym of 'rot')
'green politician in party starts to spout environmental' is the wordplay.
'green' becomes 'eco' (abbreviation for ecological).
'politician' becomes 'mp'.
'in' is an insertion indicator.
'party' becomes 'do' (do is a kind of party).
'starts to' suggests taking the first letters.
The first letters of 'spout environmental' is 'se'.
'eco'+'mp'='ecomp'
'ecomp' going into 'do' is 'decompo'.
'decompo'+'se'='DECOMPOSE'
(Other definitions for decompose that I've seen before include "Rot" , "Break down" , "Decay, disintegrate" , "Crumble" , "as rotters do!" .)