Derision, not starting cereal (4)
I believe the answer is:
corn
'cereal' is the definition.
(I know that corn is a type of cereal)
'derision not starting' is the wordplay.
'derision' becomes 'scorn' (I've seen this in another clue).
'not starting' means to remove the first letter.
'scorn' with its initial letter removed is 'CORN'.
(Other definitions for corn that I've seen before include "Grain on a toe?" , "Trite jokes" , "Maize" , "Type of cereal" , "Hard thickening of the skin on a toe" .)