The most common and most likely answer to this crossword clue is the 4 letter word TSAR
I've seen this crossword clue in The Times
T
S
A
R
More clues leading to the result TSAR
- Title — Serbia’s and Russia’s leaders carry it
- Russian leader succeeded in turning informer
- Ex-leader's requests are largely ignored
- Son breaking sailor's old ruler
- Old ruler head of school dropped in flipping grass
- Rastafarian leaders backing former emperor
- Some uprising from Caesar, a stonking emperor!
- Originally the supreme autocrat, leader of Russia
- Emperor of eastern half of Sikh city
- Artist, good person set up as policy chief
Buddy explains!

The straight part
We know that "Ex-leader" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ex-leader = tsar.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 TSAR mean?
TSAR noun- An emperor of Russia (1547 to 1917) and of some South Slavic states.
- A person with great power; an autocrat.