Noticing nothing odd in disguise (9)
I believe the answer is:
incognito
'in disguise' is the definition.
(I've seen this before)
'noticing nothing odd' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'odd' indicates anagramming the letters.
'noticing'+'o'='noticingo'
'noticingo' with letters rearranged gives 'INCOGNITO'.
(Other definitions for incognito that I've seen before include "as disguised person appears?" , "With identity hidden" , "Camouflaged" , "Keeping one's identity a secret" , "On the sly" .)