The most common and most likely answer to this crossword clue is the 4 letter word AINU
I've seen this crossword clue in The Guardian
A
I
N
U
Buddy explains!
The straight part
We know that "Indigenous people from area in eastern part of Honshu" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case indigenous people from area in eastern part of honshu = ainu.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.