Pupil guiltily holds back fruit (4)
I believe the answer is:
ugli
'fruit' is the definition.
(ugli is a kind of fruit)
'pupil guiltily holds back' is the wordplay.
'holds' says the answer is hidden in the clue.
'back' shows that the letters should be reversed in order.
'ilgu' is hidden within 'pupil guiltily'.
'ilgu' backwards is 'UGLI'.
(Other definitions for ugli that I've seen before include "Citrus fruit of the West Indies" , "Sweet variety of tangelo from Jamaica" , "Large juicy fruit with thick wrinkled skin" , "Hybrid of tangerine and grapefruit" , "Mottled tangelo hybrid" .)