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