Belfast's province (6)
I believe the answer is:
ulster
'belfast's province' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ulster that I've seen before include "Six, or nine, counties" , "Area of Ireland" , "Northern Ireland (loosely)" , "Region of Ireland" , "Overcoat; former kingdom in Ireland" .)