Largest Japanese island (6)
I believe the answer is:
honshu
'largest japanese island' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for honshu that I've seen before include "Island regarded as the Japanese mainland" , "Largest of the Japanese islands" , "part of Japan" , "Largest of the islands forming Japan" , "Largest island of mainland Japan" .)