Sneer at last of this cereal (5)
I believe the answer is:
scorn
'sneer at' is the definition.
(thesaurus)
'last of this cereal' is the wordplay.
'last of' suggests the final letters.
'cereal' becomes 'corn' (corn is a kind of cereal).
The last letter of 'this' is 's'.
's'+'corn'='SCORN'
(Other definitions for scorn that I've seen before include "Extreme contempt" , "Open disrespect" , "Look at with disdain" , "Disdain or derision" , "Spurn" .)