Rum and Asti I mixed in dessert (8)
I believe the answer is:
tiramisu
'dessert' is the definition.
(I've seen this before)
'rum and asti i mixed' is the wordplay.
'and' says to put letters next to each other.
'mixed' is an anagram indicator.
'asti'+'i'='astii'
'rum' put after 'astii' is 'astiirum'.
'astiirum' is an anagram of 'TIRAMISU'.
'in' acts as a link.
(Other definitions for tiramisu that I've seen before include "for which damp sponge is needed" , "Arenaceous material" , "Dessert soaked in alcohol" , "It. dessert, literally pick me up" , "Italian dessert of cake, coffee and chocolate" .)