Brunei's island (6)
I believe the answer is:
borneo
'brunei's island' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for borneo that I've seen before include "The world's third-largest island" , "Oberon (anag) - island" , "Largest island of the Malay archipelago" , "Largest island of Asia" , "West Pacific island" .)