Fen, swamp (5)
I believe the answer is:
marsh
I believe this clue is a double definition.
'fen' is the first definition.
(I know that fen can be written as marsh)
'swamp' is the second definition.
(I've seen this before)
(Other definitions for marsh that I've seen before include "eg Romney" , "Swampland" , "Fen" , "Swampy ground" , "Low-lying wet land with grassy vegetation" .)