In disguise, strangely noticing nothing (9)
I believe the answer is:
incognito
'in disguise' is the definition.
(I've seen this before)
'strangely noticing nothing' is the wordplay.
'strangely' indicates anagramming the letters.
'nothing' becomes 'O' (O resembles 0).
'noticing' with letters rearranged gives 'incognit'.
'incognit'+'o'='INCOGNITO'
(Other definitions for incognito that I've seen before include "Using false name" , "Camouflaged" , "appear unidentifiable" , "Under assumed identity" , "hiding identity" .)