When mate's most likely to kill grouse? (7)
I believe the answer is:
endgame
'when mate's most likely' is the definition.
(as in checkmate in a chess game)
'kill grouse?' is the wordplay.
'kill' becomes 'end' (I've seen this in another clue).
'grouse?' becomes 'game' (grouse is a type of gamebird).
'end'+'game'='ENDGAME'
'to' acts as a link.
(Other definitions for endgame that I've seen before include "moves before mating" , "drama" , "closing moves on chessboard" , "final moves" , "when there are few men left" .)