Cow not left in marsh (8)
I believe the answer is:
frighten
'cow' is the definition.
(thesaurus)
'not left in marsh' is the wordplay.
'not left' becomes 'right' ('right' is the opposite to 'left').
'in' means one lot of letters goes inside another.
'marsh' becomes 'fen' ('fen' can be a synonym of 'marsh').
'right' placed within 'fen' is 'FRIGHTEN'.
(Other definitions for frighten that I've seen before include "Shock" , "Put the wind up" , "Cause fear" , "Terrify" , "Spook" .)