Pool, lake (4)
I believe the answer is:
mere
I believe 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 pond)
(Other definitions for mere that I've seen before include "Nothing more than a small pool" , "Lake - French mother" , "Nothing more than is specified - or a poetic lake" , "Bare, simple" , "President Hollande's mother?" .)