The most common and most likely answer to this crossword clue is the 7 letter word DESPOTS
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More clues leading to the result DESPOTS
Buddy explains!

The straight part
We know that "tyrannical rulers" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tyrannical rulers = despots.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 DESPOTS mean?
DESPOTS noun- A ruler with absolute power; a tyrant.
- A title awarded to senior members of the imperial family in the late Byzantine Empire, and claimed by various independent or semi-autonomous rulers in the Balkans (12th to 15th centuries)