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 (bore can mean to drill into).
'a' becomes 'per' (eg 'one a day' means 'one per day').
'lay' inserted within '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" .)