The most common and most likely answer to this crossword clue is the 5 letter word SIGMA
S
I
G
M
A
More clues leading to the result SIGMA
- Foreign character shortly put name to parent
- Soldier’s brought back mother’s letter from Greece
- Athenian character's black mark for wasting time
- Little time to avoid damage to reputation for Greek character
- Disgraceful mark time erased in letter to Greeks
- What sounds like a fag, postgraduate producing letter in Greek
- Letter making case for Sophocles?
- Rise for soldier's mother, a character in Thessaloniki
- US soldiers sent back mum’s letter from abroad
- The end of Athens, as described by Homer?
Buddy explains!

The straight part
We know that "Greek letter/" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case greek letter/ = sigma.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 SIGMA mean?
SIGMA noun- The eighteenth letter of the Classical and Modern Greek alphabets, the twentieth letter of Old and Ancient.
- The symbol Σ, used to indicate summation of a set or series.
- The symbol σ, used to indicate one standard deviation from the mean, particularly in a normal distribution.