Sultanate on Borneo (6)
I believe the answer is:
brunei
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sultanate on borneo</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)