Staunch pal's defeat (9)
I believe the answer is:
'defeat' is the definition.
(checkmating is a kind of defeating)
'staunch pal's' is the wordplay.
'staunch' becomes 'check' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'pal' becomes 'mate' (I've seen this before).
Can you help me to learn more?
(Other definitions for checkmate that I've seen before include "position of defeat" , "final defeat" , "End of game" , "Conclusion of chess game" , "The final winning move in chess" .)