A western missing out on Oscar is maybe flat (10)
I believe the answer is:
accidental
'is maybe flat' is the definition.
(flat is a type of accidental note in music)
'a western missing out on oscar' is the wordplay.
'western' becomes 'occidental' (similar in meaning).
'missing out on' suggests deleting specific letters.
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'occidental' with 'o' removed is 'ccidental'.
'a'+'ccidental'='ACCIDENTAL'
(Other definitions for accidental that I've seen before include "Unintended - sort of note" , "not intended by composer?" , "Unintentional" , "Happening by chance" , "without planning" .)