Unexpected, a western missing out on Oscar (10)
I believe the answer is:
accidental
'unexpected' is the definition.
(thesaurus)
'a western missing out on oscar' is the wordplay.
'western' becomes 'occidental' (similar in meaning).
'missing out on' is a deletion indicator.
'oscar' becomes 'o' (phonetic alphabet: alpha, bravo, charlie etc.).
'occidental' with 'o' taken away is 'ccidental'.
'a'+'ccidental'='ACCIDENTAL'
(Other definitions for accidental that I've seen before include "Happening by chance" , "without planning" , "Unintended - sort of note" , "Sign raising or lowering a musical note" , "Chance, inadvertent" .)