No one (6)
I believe the answer is:
nobody
'no one' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for nobody that I've seen before include "Cipher" , "The Diary of a --- (George &Weedon Grossmith)" , "Not one person" , "Eg Pooter" , "The Diary of a - - (G & W Grossmith)" .)