The most common and most likely answer to this crossword clue is the 6 letter word CAESAR
I've seen this crossword clue in The Daily Telegraph
C
A
E
S
A
R
More clues leading to the result CAESAR
- Inside vehicle, the main ascendant emperor
- Dictator recalled in Marmara Sea chronicle
- Ruler’s vehicle sunk in rising sea? On the contrary!
- . Roman leader‘s vehicle circumvents sinister-looking marine
- Get that woman sounding like an emperor
- Delivery of a kind, a scare when batting
- Autocrat creating a scare
- Autocrat created a scare
- Roman emperor destroyed Anglo-Saxon race
- Crucible has first added egg sandwiches and relish to salad
Buddy explains!

The straight part
We know that "Tyrant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tyrant = caesar.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 CAESAR mean?
CAESAR noun- Emperor, ruler, dictator