Crossword Genius

This could be the apple of the teacher's eye? (5)

Ross

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" .)

I've seen this clue in The Guardian.
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