Teacher returned single snack (7)
I believe the answer is:
rissole
'snack' is the definition.
(I know that Rissole is a type of snack)
'teacher returned single' is the wordplay.
'teacher' becomes 'sir' (a male teacher might be addressed as 'sir').
'returned' is a reversal indicator.
'single' becomes 'sole' (similar in meaning).
'sir' written backwards gives 'ris'.
'ris'+'sole'='RISSOLE'
(Other definitions for rissole that I've seen before include "Fried small cake of chopped meat" , "Fried meat/breadcrumb ball" , "Lorises (anag)" , "Compressed meat and spices in breadcrumbs, fried" , "Fried meat dish" .)