Marsh conceals proper scare (8)
I believe the answer is:
frighten
'scare' is the definition.
(I know that scare can be written as frighten)
'marsh conceals proper' is the wordplay.
'marsh' becomes 'fen' (synonyms).
'conceals' is an insertion indicator.
'proper' becomes 'right' (similar in meaning).
'fen' going around 'right' is 'FRIGHTEN'.
(Other definitions for frighten that I've seen before include "Shock" , "Cow" , "Terrify" , "Scare, alarm" , "Cause fear" .)