Suspect noticing nothing in disguise (9)
I believe the answer is:
incognito
'in disguise' is the definition.
(thesaurus)
'suspect noticing nothing' is the wordplay.
'suspect' indicates an anagram.
'nothing' becomes 'o' (looks like zero - 0).
'noticing' anagrammed gives 'incognit'.
'incognit'+'o'='INCOGNITO'
(Other definitions for incognito that I've seen before include "Covertly" , "In disguise" , "under another name" , "With true identity concealed" , "Under assumed identity" .)