A professor is a handsome youngster (6)
I believe the answer is:
adonis
'a handsome youngster' is the definition.
'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 Gk. youth" , "male looker" , "one deemed to be beautiful" , "Extremely handsome young man" , "Handsome Greek loved by Aphrodite" .)