The most common and most likely answer to this crossword clue is the 6 letter word SULTAN
I've seen this crossword clue in The Daily Telegraph
S
U
L
T
A
N
More clues leading to the result SULTAN
- Muslim ruler’s last month in hospital
- American-backed Left beat ruler
- Ruler in hospital last month
- Some result – a new ruler!
- We object over student getting hit with a ruler
- A bit of cake maybe cut for a ruler
- First day during which army officer meets a ruler
- Fruit, no end, for ruler
- Ottoman leader will get no end of fruit
- It’s an insult, kind of, dropping in to see ruler
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 = sultan.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 SULTAN mean?
SULTAN noun- The holder of a secular office, formally subordinate to, but de facto the power behind the throne of, the caliph.
- A hereditary ruler in various Muslim states (sultanate), varying from petty principalities (as in Indonesia and in Yemen), often vassal of a greater ruler, to independent realms, such as Oman, Brunei, or an empire such as the Turkish Ottoman Empire.
- A variant of solitaire, played with two decks of cards.
- A breed of chicken originating in Turkey, kept primarily in gardens for ornamental reasons. See: sultan (chicken)