Nothing more than a pool (4)
I believe the answer is:
mere
This clue is a double definition.
'nothing more than' is the first definition.
(I've seen this in another clue)
'a pool' is the second definition.
(mere can mean a lake)
(Other definitions for mere that I've seen before include "Nothing more than - water" , "of no significance" , "Insignificant" , "Lake, pond (poetic)" , "Pool, small lake" .)