Amateur bores a competitor (6)
I believe the answer is:
player
'competitor' is the definition.
(I've seen this before)
'amateur bores a' is the wordplay.
'amateur' becomes 'lay' (similar in meaning).
'bores' is an insertion indicator (to bore something can mean to drill into it).
'a' becomes 'per' (eg 'one a day' means 'one per day').
'lay' put into 'per' is 'PLAYER'.
(Other definitions for player that I've seen before include "Important participant" , "Partaker in game" , "Gambler" , "Game's participant" , "Sports team member" .)