This could be the apple of the teacher's eye? (5)
I believe the answer is:
pupil
'eye?' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
(Other definitions for pupil that I've seen before include "Schoolboy or eye part" , "Eye hole" , "Learner at educational institution" , "feature in "Observer"?" , "The sort of student" .)