Sultanate on the South China Sea (6)
I believe the answer is:
brunei
'sultanate on the south china sea' 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 "Far Eastern sultanate" , "major source of oil" , "It's sultan is immensely rich" , "Sultanate of southeast Asia" , "Southeast Asian sultanate" .)