Bar on one Scots town where magistrate held sway? (9)
I believe the answer is:
bailiwick
'where magistrate held sway?' is the definition.
(area under a magistrate's jurisdiction)
'bar on one scots town' is the wordplay.
'bar' becomes 'bail' (bar on a cricket wicket).
'on' says to put letters next to each other.
'one' becomes 'I' (Roman numeral).
'scots town' becomes 'Wick' (town in Scotland).
'bail'+'i'+'wick'='BAILIWICK'
(Other definitions for bailiwick that I've seen before include "Area of interest" , "Jurisdiction of a bailiff" , "special field" , "legal official's jurisdiction" , "Jersey" .)