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