School to ridicule protective garment (5)
I believe the answer is:
smock
'protective garment' is the definition.
(smock is a kind of protective garment)
'school to ridicule' is the wordplay.
'school' becomes 'S'.
'to ridicule' becomes 'mock' (I've seen this before).
's'+'mock'='SMOCK'
(Other definitions for smock that I've seen before include "Loose outer garment" , "Artist's, peasant's, overall" , "Loose dress or blouse" , "Loose overall or dress" , "Artist's loose overall" .)