Island not entirely perfect, not entirely close either (6)
'island' is the definition.
(Honshu is an example)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for honshu that I've seen before include "Largest of the islands forming Japan", "The largest island unit of Japan", "Largest of the four main islands of Japan", "Japan's main island", "Largest island of Japan's mainland".)