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