Only a small body of water (4)
I believe the answer is:
mere
This clue is a triple definition!
'only' is the first definition.
(I've seen this in another clue)
'a small' is the second definition.
(I've seen this in another clue)
'body of water' is the third definition.
(mere is a kind of body of water)
(Other definitions for mere that I've seen before include "Nothing more than is specified - or a poetic lake" , "Just an English lake" , "Lake - French mother" , "Pool -- pure and simple" , "Lake; simple" .)