Fool: a Belfast one? (6)
I believe the answer is:
nitwit
'fool' is the definition.
(both can mean a stupid person)
'a belfast one?' is the wordplay.
'a belfast' becomes 'NI' (Belfast is the capital of Northern Ireland).
'one?' becomes 'twit' (I can't explain this - if you can you should give a lot more credence to this answer).
'ni'+'twit'='NITWIT'
(Other definitions for nitwit that I've seen before include "Simpleton" , "Turkey" , "Foolish person like humorous louse" , "Plonker" , "Birdbrain" .)