Lake discovered in game reserve (4)
I believe the answer is:
mere
'lake' is the definition.
(mere is a kind of lake)
'discovered in game reserve' is the wordplay.
'discovered' indicates a hidden word (remove the letters covering the answer in the clue).
'MERE' is hidden in the letters of 'in game reserve'.
(Other definitions for mere that I've seen before include "Nothing more than a small pond" , "sea" , "Pool, small lake" , "Pool -- pure and simple" , "Pond - pure and simple" .)