Sweet, but crotchets must be rewritten (12)
I believe the answer is:
butterscotch
'sweet' is the definition.
(butterscotch is a kind of sweet)
'but crotchets must be rewritten' is the wordplay.
'must be rewritten' indicates anagramming the letters.
'crotchets' with letters rearranged gives 'terscotch'.
'but'+'terscotch'='BUTTERSCOTCH'
(Other definitions for butterscotch that I've seen before include "Hard brittle toffee" , "Type of toffee" , "confectionery" , "Kind of toffee" , "Brittle, yellow-brown sweet" .)