Noticing nothing amiss when disguised (9)
I believe the answer is:
incognito
'disguised' is the definition.
(I've seen this in another clue)
'noticing nothing amiss' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'amiss' indicates anagramming the letters.
'noticing'+'o'='noticingo'
'noticingo' with letters rearranged gives 'INCOGNITO'.
'when' acts as a link.
(Other definitions for incognito that I've seen before include "With one's identity concealed" , "With identity hidden" , "Covertly" , "With true identity concealed" , "Without revealing one's identity" .)