Simple lake (4)
I believe the answer is:
mere
This clue is a double definition.
'simple' is the first definition.
(I know that simple can be written as mere)
'lake' is the second definition.
(I know that mere is a type of pond)
(Other definitions for mere that I've seen before include "Previously pure" , "bathing may be permitted in this?" , "Pool, small lake" , "sea" , "of no significance" .)