Staunch pal's defeat (9)
I believe the answer is:
checkmate
'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).
'check'+'mate'='CHECKMATE'
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" .)