The most common and most likely answer to this crossword clue is the 7 letter word DIGAMMA
I've seen this crossword clue in The Times
D
I
G
A
M
M
A
More clues leading to the result DIGAMMA
Buddy explains!

The straight part
We know that "Old letter" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old letter = digamma.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 DIGAMMA mean?
DIGAMMA noun- Letter of the Old Greek alphabet: Ϝ, ϝ