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