Nameless doctor noticing ring (9)
I believe the answer is:
incognito
'nameless' is the definition.
(thesaurus)
'doctor noticing ring' is the wordplay.
'doctor' indicates an anagram.
'ring' becomes 'o'.
'noticing' is an anagram of 'incognit'.
'incognit'+'o'='INCOGNITO'
(Other definitions for incognito that I've seen before include "In disguise" , "Under assumed identity" , "under another name" , "Under an assumed name" , "Using false name" .)