Pool or lake (4)
I believe the answer is:
mere
This clue is a double definition.
'pool' is the first definition.
(mere can mean a lake)
'lake' is the second definition.
(I know that mere is a type of lake)
(Other definitions for mere that I've seen before include "Nothing more than a small pool" , "Previously pure" , "Nothing more than a small pond" , "Lake in Cumbria" , "Lake; simple" .)