Only lake (4)
I believe the answer is:
mere
This clue is a double definition.
'only' is the first definition.
(I've seen this in another clue)
'lake' is the second definition.
(I know that mere is a type of lake)
(Other definitions for mere that I've seen before include "Nothing more than is specified - or a poetic lake" , "Nothing more than a small pond" , "Unimpressive" , "Bare, simple" , "of no significance" .)