Lake inside game reserve (4)

I believe the answer is:

mere

'lake' is the definition.
(mere is a kind of lake)

'inside game reserve' is the wordplay.
'inside' indicates the answer is hidden within the clue.
'MERE' can be found hidden inside 'game reserve'.

(Other definitions for mere that I've seen before include "Small area of standing water" , "sea" , "Bare, simple" , "Lake - French mother" , "Just an English lake" .)

I've seen this clue in The Times.

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