Sultanate on Borneo (6)
I believe the answer is:
brunei
'sultanate on borneo' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for brunei that I've seen before include "Sultanate in northwest Borneo" , "Oil-rich sultanate of Borneo" , "Its sultan is famously very wealthy" , "Rich sultanate in the South China Sea" , "Muslim state" .)