Japan's largest island (6)
I believe the answer is:
honshu
'japan's largest island' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for honshu that I've seen before include "part of Japan" , "Largest island of Japanese mainland" , "The main island of Japan" , "Main Japanese island" , "Largest of the four main islands of Japan" .)