Player against work with piano nonet being composed (8)
I believe the answer is:
opponent
'player against' is the definition.
(an opposing player)
'work with piano nonet being composed' is the wordplay.
'work' becomes 'op' (common abbreviation for 'opus').
'with' indicates putting letters inside.
'piano' becomes 'p' (musical abbreviation).
'being composed' indicates anagramming the letters.
'nonet' anagrammed gives 'onent'.
'p' put inside 'onent' is 'ponent'.
'op'+'ponent'='OPPONENT'
(Other definitions for opponent that I've seen before include "Foe or competitor" , "One's competitor" , "Adversary, competitor" , "member of other team" , "One competes against you" .)