A professor is handsome youth (6)
I believe the answer is:
adonis
'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 "Most 8 Greek youth" , "In Greek myth, handsome youth beloved of Aphrodite" , "Handsome youth (Gr.)" , "A very handsome legendary Greek" , "lovely man" .)