A professor is a handsome man (6)
I believe the answer is:
adonis
'a handsome man' is the definition.
(I've seen this in another clue)
'a professor is' is the wordplay.
'professor' becomes 'don' (term for a university professor).
'a'+'don'+'is'='ADONIS'
(Other definitions for adonis that I've seen before include "lovely man" , "Said no to the Greek youth" , "lovely chap" , "Most 8 Greek youth" , "Beautiful Greek youth" .)