Sultanate, British, and a French island (6)
I believe the answer is:
brunei
'sultanate' is the definition.
(Brunei is an example)
'british and a french island' is the wordplay.
'british' becomes 'br' (abbreviation for British).
'and' is a charade indicator (letters next to each other).
'a french' becomes 'une' ('a' in French).
'island' becomes 'i' (geographical abbreviation).
'br'+'une'+'i'='BRUNEI'
(Other definitions for brunei that I've seen before include "Sultanate in northwest Borneo" , "E. Indies sultanate" , "Commonwealth member" , "Oil-rich sultanate of Borneo" , "Rich sultanate in the South China Sea" .)