Handsome youth is behind a teacher (6)
I believe the answer is:
adonis
'handsome youth' is the definition.
(I've seen this before)
'is behind a teacher' is the wordplay.
'behind' says to put letters next to each other.
'teacher' becomes 'don' (don is a kind of teacher).
'a'+'don'='adon'
'is' after 'adon' is 'ADONIS'.
(Other definitions for adonis that I've seen before include "one deemed to be beautiful" , "dandy" , "Handsome youth - said no (anag)" , "Extremely attractive young man from Greek mythology" , "handsome lad" .)