Belfast's county (6)
I believe the answer is:
antrim
'belfast's county' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for antrim that I've seen before include "County of northeast Ireland" , "town in Ireland" , "Irish county of musically green glens" , "N Ireland county of green glens" , "Giant's Causeway county" .)