With identity concealed, noticing drunk needs oxygen (9)
I believe the answer is:
incognito
'with identity concealed' is the definition.
(I've seen this before)
'noticing drunk needs oxygen' is the wordplay.
'drunk' is an anagram indicator.
'needs' says to put letters next to each other.
'oxygen' becomes 'O' (atomic symbol for oxygen).
'noticing' is an anagram of 'incognit'.
'incognit'+'o'='INCOGNITO'
(Other definitions for incognito that I've seen before include "With true identity concealed" , "Covertly" , "Camouflaged" , "under another name" , "Under assumed identity" .)