Teacher returned single croquette (7)
I believe the answer is:
rissole
'croquette' is the definition.
(I've seen this before)
'teacher returned single' is the wordplay.
'teacher' becomes 'sir' (I've seen this before).
'returned' is a reversal indicator.
'single' becomes 'sole' ('sole' can be a synonym of 'single'**).
'sir' reversed gives 'ris'.
'ris'+'sole'='RISSOLE'
(Other definitions for rissole that I've seen before include "Small cake of deep-fried minced meat" , "Minced meat or fish deep fried in egg and breadcrumbs" , "Fried meat dish" , "Fried and breadcrumbed dish" , "Meatball fried in crumbs" .)