Sultanate, British, and a French one (6)
I believe the answer is:
brunei
'sultanate' is the definition.
(Brunei is an example)
'british and a french one' is the wordplay.
'british' becomes 'br'.
'and' is a charade indicator (letters next to each other).
'a french' becomes 'une' ('a' in French).
'one' becomes 'i' (Roman numeral).
'br'+'une'+'i'='BRUNEI'
(Other definitions for brunei that I've seen before include "State on Borneo" , "major source of oil" , "Rue bin in the sultanate" , "Rich sultanate in the South China Sea" , "Ill" .)