"Who ___?" (4)
'who?' is the definition.
(from 'Doctor Who', the famous TV series)
This is all the clue.
(Other definitions for doctor that I've seen before include "Tamper with - falsify - one of the 14 films" , "Onshore breeze" , "Foster, Who, Jekyll or Livingstone" , "Medical professional or university don" , "Physician" .)
(Other definitions for else that I've seen before include "As an alternative" , "Other than (what is under consideration)" , "In addition, besides" , "Adverb meaning if not, otherwise" , "Eels (anag)" .)
(Other definitions for knew that I've seen before include "Understood" , "Was familiar with" , "Had learnt; sounds like modern" , "Distinguished" , "Acknowledged" .)
(Other definitions for says that I've seen before include "Expresses in words" , "Verbalises" , "States" , "Utters" , "Traditional Mexican strolling musician" .)
(Other definitions for isnt that I've seen before include "Is not (colloq.)" , "Ain't" , "In the present, not to be" , "Is not (abbrev.)" , "Does not happen to be" .)
(Another definition for sez that I've seen is " Says (slang)".)