The most common and most likely answer to this crossword clue is the 10 letter word PREFECTURE
I've seen this crossword clue in Financial Times
P
R
E
F
E
C
T
U
R
E
Buddy explains!
The straight part
We know that "part of Japan" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case part of japan = prefecture.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.
What does PREFECTURE mean?
PREFECTURE noun- The office or position of a prefect.
- The jurisdiction of a prefect; the region administered by a prefect, especially as a translation of certain French, Chinese, and Japanese administrative divisions.