Noticing nothing strange in disguise (9)
I believe the answer is:
incognito
'in disguise' is the definition.
(thesaurus)
'noticing nothing strange' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'strange' indicates anagramming the letters.
'noticing'+'o'='noticingo'
'noticingo' with letters rearranged gives 'INCOGNITO'.
(Other definitions for incognito that I've seen before include "under another name" , "On the sly" , "With one's identity concealed" , "With identity kept secret, perhaps travelling thus" , "Keeping one's identity a secret" .)