Spy on partner as one's final move (9)
I believe the answer is:
checkmate
'one's final move' is the definition.
(final move in chess)
'spy on partner' is the wordplay.
'spy on' becomes 'check' (check up on someone).
'partner' becomes 'mate' (synonyms).
'check'+'mate'='CHECKMATE'
'as' is the link.
(Other definitions for checkmate that I've seen before include "ending on board" , "final defeat" , "Where the king can't move" , "Declaration of chess victory" , "End of game" .)