Gin and tonic drunk by Oscar in disguise (9)
I believe the answer is:
incognito
'in disguise' is the definition.
(I've seen this before)
'gin and tonic drunk by oscar' is the wordplay.
'and' says to put letters next to each other.
'drunk' indicates an anagram.
'by' is a charade indicator (letters next to each other).
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'gin' put after 'tonic' is 'tonicgin'.
'tonicgin' is an anagram of 'incognit'.
'incognit'+'o'='INCOGNITO'
(Other definitions for incognito that I've seen before include "With true identity concealed" , "under another name" , "Anonymously" , "On the sly" , "Using false name" .)