A professor is a handsome lad (6)
I believe the answer is:
adonis
'a handsome lad' 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 "handsome lad" , "Legendary lover" , "A very good-looking ancient Greek" , "Hunk" , "Beautiful Greek youth" .)