Marshy area (3)
I believe the answer is:
fen
'marshy area' is the definition.
(both can mean wetland)
This is all the clue.
(Other definitions for fen that I've seen before include "One hundredth of a yuan" , "Low marshy land" , "Marshy area of land" , "Low-lying marshy area in England" , "Slough" .)