The most common and most likely answer to this crossword clue is the 3 letter word RHO
I've seen this crossword clue in The Times
R
H
O
More clues leading to the result RHO
- Letter Cecil only half finished
- Auditor’s rank that’s used for a spell in Greece
- With right hand, start to open letter from abroad
- Greek character from western half of island
- Character in Iliad oddly ignored arch-foe
- Character from Athens — another Homer? Not entirely
- The P of Patras in Greece
- Something like 25 occupying another hotel
- Girl ignoring lawyer’s letter
- Greek character in series on the radio
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 = rho.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 RHO mean?
RHO noun- The seventeenth letter of the Modern Greek and Classical alphabets and the nineteenth letter of Old and Ancient.
- The sensitivity of the option value to the risk-free interest rate.