The most common and most likely answer to this crossword clue is the 7 letter word KAISERS
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Buddy explains!

The straight part
We know that "rulers (German emperors)" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rulers (german emperors) = kaisers.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 KAISERS mean?
KAISERS noun- An emperor of a German-speaking country, particularly the Holy Roman Empire (962–1806), the Austrian Empire (1806–1918), or the German Empire (1871–1918) — often specifically Wilhelm II — or any emperor.
- A Kaiser roll.
- A person who exercises or tries to exercise absolute authority; autocrat.