Only a butter lake, perhaps? (4)

I believe the answer is:

mere

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'only' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Other definitions for mere that I've seen before include "Lake - French mother" , "Nothing more than is specified - or a poetic lake" , "of no significance" , "Pierre's mother identified" , "Former lake" .)

I've seen this clue in the Evening Standard.

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