A professor is a handsome youth (6)
I believe the answer is:
adonis
'a handsome youth' is the definition.
(I've seen this before)
'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 chap" , "Hunk" , "legendary Greek noted for good looks" , "Legendary lover" , "Extremely attractive young man from Greek mythology" .)