Sultanate on the South China Sea (6)
I believe the answer is:
brunei
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sultanate on the south china sea</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 "Far Eastern sultanate" , "major source of oil" , "It's sultan is immensely rich" , "Sultanate of southeast Asia" , "Southeast Asian sultanate" .)