The most common and most likely answer to this crossword clue is the 4 letter word SHAH
I've seen this crossword clue in The Sunday Telegraph
S
H
A
H
More clues leading to the result SHAH
- Old Persian ruler’s quiet expression of pleasure
- Keep quiet about a Persian ruler
- Persian ruler’s quiet expression of satisfaction
- Head of state?
- Former ruler making mess – rebels ultimately to the fore
- Persian ruler shut up I see
- In Bangladesh, a Hindu ruler once
- Former ruler‘s debacle, allowing son to take precedence
- Quiet expression of scepticism about leader
- Ruler has worked hard
Buddy explains!

The straight part
We know that "ruler once" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ruler once = shah.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 SHAH mean?
SHAH noun- A king of Persia or Iran.
- A supreme ruler in some Middle Eastern or South Asian nations.
- A Ukrainian monetary unit.