The most common and most likely answer to this crossword clue is the 5 letter word KAPPA
I've seen this crossword clue in The Times
K
A
P
P
A
More clues leading to the result KAPPA
- Greek character in dark apparel
- Finally hack program with a character from abroad
- Letter from abroad found in sack apparently
- One’s told to seal a letter to Zorba
- Letter for Mowgli’s snake held very quietly
- Finally hack program with a letter from abroad
- Character in Greek apparel
- Greek letter, a couple of pages in the spirit of an Egyptian
- “Pick me up” says a Greek character
- Letter from Greece united Kelvin with father in Chennai
Buddy explains!

The straight part
We know that "Greek character" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case greek character = kappa.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 KAPPA mean?
KAPPA noun- The tenth letter of the Greek alphabet.
- A measurement of the sensitivity of the value of an option to changes in the implied volatility of the price of the underlying asset.
- A tortoise-like creature in the Japanese mythology.