Surplus cake in Ulster? (8)
I believe the answer is:
overcoat
'ulster?' is the definition.
(ulster is a kind of overcoat)
'surplus cake' is the wordplay.
'surplus' becomes 'over' (I've seen this before).
'cake' becomes 'coat' ('coat' can be a synonym of 'cake'**).
'over'+'coat'='OVERCOAT'
'in' is the link.
(Other definitions for overcoat that I've seen before include "Ulster, for example" , "Protective winter garment" , "Warm garment" , "Outdoor garment" , "Heavy outer garment" .)