End of a chess game (9)
(Other definitions for checkmate that I've seen before include "Defeat totally" , "Chess game ending" , "Inescapable attack in chess" , "defeat on board" , "you lose!" .)
'end of a chess game' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mate that I've seen before include "couple" , "Team (anag.)" , "Finish game" , "Sailor" , "When one can no longer play" .)