Largest of the islands that constitute Japan (6)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
honshu
(Other definitions for honshu that I've seen before include "The largest island unit of Japan" , "Largest island of mainland Japan" , "Largest island of Japanese mainland" , "Largest of the islands forming Japan" , "Island regarded as the Japanese mainland" .)