Noticing changes, getting round in disguise (9)
I believe the answer is:
incognito
'in disguise' is the definition.
(thesaurus)
'noticing changes getting round' is the wordplay.
'changes' indicates an anagram.
'getting round' becomes 'o' (I've seen this in other clues).
'noticing' is an anagram of 'incognit'.
'incognit'+'o'='INCOGNITO'
(Other definitions for incognito that I've seen before include "With one's identity concealed" , "with unknown identity" , "Covertly" , "Keeping one's identity a secret" , "as something else" .)