An oversight, perhaps, Altman's first western not getting Oscar (10)
I believe the answer is:
accidental
'an oversight perhaps' is the definition.
I can't judge whether this defines the answer.
'altman's first western not getting oscar' is the wordplay.
'altman's first' becomes 'a' (1st letter of 'altman').
'western' becomes 'occidental' (similar in meaning).
'not getting' indicates named letters should be taken away.
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'occidental' with 'o' removed is 'ccidental'.
'a'+'ccidental'='ACCIDENTAL'
Can you help me to learn more?
(Other definitions for accidental that I've seen before include "Possible sharp" , "change of key" , "Unintended - sort of note" , "Subsidiary" , "Chance, inadvertent" .)