A teacher 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 teacher is' is the wordplay.
'teacher' becomes 'don' (don is a kind of teacher).
'a'+'don'+'is'='ADONIS'
(Other definitions for adonis that I've seen before include "dandy" , "legendary Greek noted for good looks" , "Handsome youth - boy from Greek mythology" , "In Greek myth, handsome youth beloved of Aphrodite" , "A very handsome ancient Greek" .)