Sweet rum and Asti I mixed up (8)
I believe the answer is:
tiramisu
'sweet' is the definition.
(type of dessert or sweet)
'rum and asti i mixed up' is the wordplay.
'and' says to put letters next to each other.
'mixed up' indicates an anagram.
'rum'+'asti'+'i'='rumastii'
'rumastii' anagrammed gives 'TIRAMISU'.
(Other definitions for tiramisu that I've seen before include "Arenaceous material" , "Italian sponge pudding" , "Italian course" , "for which damp sponge is needed" , "Italian sponge dessert" .)