Simple - pond (4)
I believe the answer is:
mere
I believe this is a double definition.
'simple' is the first definition.
(I know that simple can be written as mere)
'pond' is the second definition.
(I know that mere is a type of pond)
(Other definitions for mere that I've seen before include "Lake; simple" , "Former lake" , "Pierre's mother identified" , "President Hollande's mother?" , "Without additions or modifications" .)