Be contemptuous of sulphur put on cereal (5)
I believe the answer is:
scorn
'be contemptuous of' is the definition.
(thesaurus)
'sulphur put on cereal' is the wordplay.
'sulphur' becomes 'S' (synonyms).
'put on' is a charade indicator (letters next to each other).
'cereal' becomes 'corn' (corn is a kind of cereal).
's'+'corn'='SCORN'
(Other definitions for scorn that I've seen before include "Reject contemptuously" , "Look at with disdain" , "Deride, show contempt for" , "Mocking contempt" , "Open contempt, disdain" .)