Dunce's dessert (4)
I believe the answer is:
fool
This clue is a double definition.
'dunce's' is the first definition.
(both can mean a stupid person)
'dessert' is the second definition.
(type of dessert)
(Other definitions for fool that I've seen before include "` `More people know Tom .... that Tom .... knows' '" , "Trifle" , "Dupe - dessert" , "Charlie" , "Empty-headed person" .)