Unhappy about a pupil's food (5)
I believe the answer is:
salad
'food' is the definition.
(salad is a kind of food)
'unhappy about a pupil's' is the wordplay.
'unhappy' becomes 'sad' (synonyms).
'about' is an insertion indicator.
'pupil' becomes 'L' (as in a learner driver with L-plates on their car).
'a'+'l'='al'
'sad' placed around 'al' is 'SALAD'.
(Other definitions for salad that I've seen before include "See 22" , "Food dish" , "One may well be green" , "rabbit food" , "A dish of lettuce and vegetables" .)