Oscar, a loud idiot (3)
I believe the answer is:
oaf
'idiot' is the definition.
(I've seen this in another clue)
'oscar a loud' is the wordplay.
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'loud' becomes 'f' (short for forte - 'loud' in music).
'o'+'a'+'f'='OAF'
(Other definitions for oaf that I've seen before include "Barbarian" , "Awkward stupid person" , "Lout; dolt" , "Lout, lummox" , "Boor or lout" .)