The most common and most likely answer to this crossword clue is the 6 letter word BANZAI
I've seen this crossword clue in The Guardian
B
A
N
Z
A
I
More clues leading to the result BANZAI
Buddy explains!

The straight part
We know that "Long live the Emperor" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case long live the emperor = banzai.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 BANZAI mean?
BANZAI noun- A cry or cheer of "banzai", to express enthusiasm or celebrate victory.
- A short form of banzai attack or banzai charge.
- Thrill-seeking; wild.
- A cry or cheer of enthusiasm, or to celebrate victory.