Only a stretch of water (4)
I believe the answer is:
mere
I believe this clue is a double definition.
'only' is the first definition.
(I've seen this before)
'a stretch of water' is the second definition.
(literary term for a lake)
(Other definitions for mere that I've seen before include "sea" , "Pond - pure and simple" , "Pool, small lake" , "Lake (and nothing else?)" , "Paltry" .)