Staunch pal's defeat on board (9)
I believe the answer is:
checkmate
'defeat on board' is the definition.
(I've seen this before)
'staunch pal's' is the wordplay.
'staunch' becomes 'check' (I can't justify this - if you can you should believe this answer much more).
'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 "Winning call on board" , "Inescapable attack in chess" , "Chess game ending" , "game's up now" , "Where the king can't move" .)