Doctor noticing Oscar's assumed identity (9)
I believe the answer is:
incognito
'assumed identity' is the definition.
The answer and definition are not the same part of speech. However, adjectives and past participle verbs can sometimes define each other.
'doctor noticing oscar's' is the wordplay.
'doctor' indicates an anagram.
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'noticing' anagrammed gives 'incognit'.
'incognit'+'o'='INCOGNITO'
Can you help me to learn more?
(Other definitions for incognito that I've seen before include "With identity kept secret, perhaps travelling thus" , "Keeping one's identity a secret" , "In disguise" , "as disguised person appears?" , "appear unidentifiable" .)