The most common and most likely answer to this crossword clue is the 4 letter word ETAS
I've seen this crossword clue in The Times
E
T
A
S
Buddy explains!

The straight part
We know that "Greek characters" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case greek characters = etas.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 ETAS mean?
ETAS noun- The seventh letter of the Modern Greek alphabet, the eighth in Old Greek.
- A kind of electrically neutral meson having zero spin and isospin.
- A social outcast in Japan who is subjected to menial work, making up a class or caste of such people.