Concealed nothing after gin and tonic's drunk (9)
I believe the answer is:
incognito
'concealed' is the definition.
(thesaurus)
'nothing after gin and tonic's drunk' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'after' is a charade indicator (letters next to each other).
'and' means one lot of letters go next to another (I've seen this in other clues).
'drunk' indicates an anagram.
'gin' put after 'tonic' is 'tonicgin'.
'tonicgin' is an anagram of 'incognit'.
'o' put after 'incognit' is 'INCOGNITO'.
(Other definitions for incognito that I've seen before include "Under assumed identity" , "With identity hidden" , "Without revealing one's identity" , "With one's identity concealed" , "With identity kept secret, perhaps travelling thus" .)