Lake or marsh (4)
I believe the answer is:
mere
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">lake</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that mere is a type of lake)</span><br/ ><br/ >'<span class="explanation-format__original">marsh</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ ></span>
(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" .)