A professor is a handsome young man (6)
I believe the answer is:
adonis
'a handsome young 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 "Handsome young man (of Greek myth)" , "A very good-looking ancient Greek" , "Aphrodite's love" , "dandy" , "A very handsome legendary Greek" .)