'Marsh crushes political faction' scare (8)
I believe the answer is:
frighten
'scare' is the definition.
(I know that scare can be written as frighten)
'marsh crushes political faction' is the wordplay.
'marsh' becomes 'fen' (synonyms).
'crushes' means one lot of letters goes inside another.
'political faction' becomes 'right' (rightwing).
'fen' placed around 'right' is 'FRIGHTEN'.
(Other definitions for frighten that I've seen before include "Cause alarm" , "Spook" , "Scare, alarm" , "Cause fear" , "Cow" .)