The most common and most likely answer to this crossword clue is the 8 letter word TETRARCH
I've seen this crossword clue in Independent
T
E
T
R
A
R
C
H
More clues leading to the result TETRARCH
Buddy explains!

The straight part
We know that "ruler" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ruler = tetrarch.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 TETRARCH mean?
TETRARCH noun- A governor of part of a country, especially of a fourth part of a province in Ancient Rome
- An officer in charge of a fourth part of a phalanx in Ancient Greece