Get anxious just within marshy area (8)
I believe the answer is:
frighten
'get anxious' is the definition.
(frightening someone might make them anxious)
'just within marshy area' is the wordplay.
'just' becomes 'right' (associated in meaning).
'within' means one lot of letters goes inside another.
'marshy area' becomes 'fen' (both can mean wetland).
'right' going within 'fen' is 'FRIGHTEN'.
(Other definitions for frighten that I've seen before include "Cause alarm" , "Terrify" , "Spook" , "Shock" , "Cause fear" .)