Main island of Japan (6)
I believe the answer is:
honshu
'main island of japan' 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 "Main Japanese island" , "Largest of the islands forming Japan" , "Largest of the Japanese islands" , "The largest island unit of Japan" , "Largest of the four main islands of Japan" .)