Immorality about Oscar’s speech (5)
I believe the answer is:
voice
'speech' is the definition.
(I've seen this before)
'immorality about oscar's' is the wordplay.
'immorality' becomes 'vice' (vice can mean immoral behaviour).
'about' is an insertion indicator.
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'vice' enclosing 'o' is 'VOICE'.
(Other definitions for voice that I've seen before include "Give expression to" , "Sound produced by humans" , "Express, utter" , "Wallop" , "Computerised analysis of speech" .)