The most common and most likely answer to this crossword clue is the 3 letter word XIS
X
I
S
More clues leading to the result XIS
Buddy explains!

The straight part
We know that "signs from Greece" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case signs from greece = xis.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 XIS mean?
XIS noun- The 14th letter of Classical and Modern Greek. The 15th in Ancient and Old Greek.
- Either of a pair of hyperons having spin 1/2, which decay into a lambda particle and a pion.