It's only water (4)
I believe the answer is:
mere
This clue is a double definition.
'it's only' is the first definition.
(I've seen this before)
'water' is the second definition.
(I know that mere is a type of pond)
(Other definitions for mere that I've seen before include "Bare, simple" , "Lake (and nothing else?)" , "DDwater" , "sea" , "Lake; Wiltshire town" .)