Lake or marsh (4)
I believe the answer is:
mere
I believe this clue is a double definition.
'lake' is the first definition.
(I know that mere is a type of lake)
'marsh' is the second definition.
(I've seen this in another clue)
(Other definitions for mere that I've seen before include "Just an English lake" , "Pond - pure and simple" , "bathing may be permitted in this?" , "Pure; lake" , "Previously pure" .)