Only a pool (4)
I believe the answer is:
mere
I believe this is a double definition.
'only' is the first definition.
(I've seen this before)
'a pool' is the second definition.
(mere can mean a lake)
(Other definitions for mere that I've seen before include "Lake; Wiltshire town" , "Bare, simple" , "Lake (poetic); Maori war club" , "of no significance" , "Nothing more than what is specified (4)" .)