The most common and most likely answer to this crossword clue is the 4 letter word OGAM
O
G
A
M
More clues leading to the result OGAM
Buddy explains!
The straight part
We know that "inscription that’s familiar to Celts" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case inscription that’s familiar to celts = ogam.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 OGAM mean?
OGAM noun- A single character in this alphabet.