Nothing but a lake (4)
I believe the answer is:
mere
This clue is a double definition.
'nothing but' is the first definition.
(I've seen this in another clue)
'a lake' is the second definition.
(I know that mere is a type of pond)
(Other definitions for mere that I've seen before include "Small area of standing water" , "a drop of water" , "Nothing more than a small pond" , "bathing may be permitted in this?" , "Pond, only what is said" .)