Ultimately lose weight, getting room for eating dessert (4,4)

'dessert' is the definition.
(type of dessert or pudding)

'ultimately lose weight getting room for eating' is the wordplay.
'ultimately' suggests the final letters.
'weight' becomes 'ton' (unit of weight).
'getting' means one lot of letters go next to another.
'room for eating' becomes 'mess' (eg in the army).
The final letter of 'lose' is 'e'.
'e'+'ton'+'mess'='ETON MESS'