Doesn’t eat quickly at school (5)
I believe the answer is:
fasts
'doesn't eat' is the definition.
(I've seen this in another clue)
'quickly at school' is the wordplay.
'quickly' becomes 'fast' (synonyms).
'at' is a charade indicator (letters next to each other).
'school' becomes 'S'.
'fast'+'s'='FASTS'
(Other definitions for fasts that I've seen before include "Avoids food" , "Times of abstinence" , "Goes without food" , "Goes hungry" , "Abstains from food" .)