Simple body of water (4)
I believe the answer is:
mere
I believe this clue is a double definition.
'simple' is the first definition.
('mere' can be a synonym of 'simple')
'body of water' is the second definition.
(mere is a kind of body of water)
(Other definitions for mere that I've seen before include "Previously pure" , "President Hollande's mother?" , "Nothing more than a small pool" , "Simply, nothing more than" , "DDwater" .)