The most common and most likely answer to this crossword clue is the 7 letter word OMICRON
O
M
I
C
R
O
N
More clues leading to the result OMICRON
- Greek character, very small, working outside
- Greek character featured by comic, Ronnie
- Greek letter regarding housing crisis initially framed by artist
- Entering club, champagne primarily on order for Greek character
- Greek character, attention-seeker captivating male friend, briefly
- Order panel for church, enclosing rector's last letter
- Greek character’s order, constant in club
- Skip introduction to funny little man, a foreign character
- Order press to cover key letter from Plato
- Round character very short in leg
Buddy explains!

The straight part
We know that "letter in Greek" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case letter in greek = omicron.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 OMICRON mean?
OMICRON noun- The 15th letter of Classical and Modern Greek, and the 16th in Ancient and Old Greek.