Quietly film participant in sport (6)
I believe the answer is:
player
'participant in sport' is the definition.
(I've seen this before)
'quietly film' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'film' becomes 'layer' (I've seen this before).
'p'+'layer'='PLAYER'
(Other definitions for player that I've seen before include "Sportsman - instrumentalist" , "Sports participant" , "Participant in competitive game" , "Sports team member" , "Gamer or sportsman" .)