Just a lake (4)
I believe the answer is:
mere
This clue is a double definition.
'just' is the first definition.
('just' can mean 'mere' or 'only')
'a lake' is the second definition.
(I know that mere is a type of lake)
(Other definitions for mere that I've seen before include "Pure; lake" , "bathing may be permitted in this?" , "Lake, pond (poetic)" , "Pool -- pure and simple" , "See 13" .)