Musician’s piano course (6)
I believe the answer is:
player
'musician's' is the definition.
(synonyms)
'piano course' is the wordplay.
'piano' becomes 'p' (musical abbreviation).
'course' becomes 'layer' (I've seen this before).
'p'+'layer'='PLAYER'
(Other definitions for player that I've seen before include "One taking part in a game" , "Sportsperson; actor" , "Sports participant" , "one on the wing, perhaps?" , "Game's participant" .)