Dessert, one with butter, is in belly that needs reduction (8)
I believe the answer is:
tiramisu
'dessert one with butter is in belly that needs reduction' is the definition.
'tiramisu' can be an answer for 'dessert' (I've seen this before). I am not sure about the rest of the definition.
'dessert one with butter is in' is the wordplay.
'dessert one' becomes 'tiu' (I can't justify this - if you can you should believe this answer much more).
'with' indicates putting letters inside.
'butter' becomes 'ram' (a ram is something which 'butts').
'in' indicates putting letters inside.
'tiu' placed around 'ram' is 'tiramu'.
'tiramu' enclosing 'is' is 'TIRAMISU'.
Can you help me to learn more?
(Other definitions for tiramisu that I've seen before include "Italian sponge pudding" , "Sweet" , "I must air the Italian dessert" , "Dessert -- is it arum? (anag)" , "It. dessert, literally pick me up" .)