Just a body of water (4)

I believe the answer is:

mere

I believe this clue is a double definition.

'just' is the first definition.
('just' can mean 'mere' or 'only')

'a body of water' is the second definition.
(mere is a kind of body of water)

(Other definitions for mere that I've seen before include "Pierre's mother identified" , "Insignificant" , "Former lake" , "Lake, pond (poetic)" , "Pond - pure and simple" .)

I've seen this clue in the Evening Standard and The Telegraph.

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