The most common and most likely answer to this crossword clue is the 10 letter word SCRIVENERS
I've seen this crossword clue in Financial Times
S
C
R
I
V
E
N
E
R
S
More clues leading to the result SCRIVENERS
Buddy explains!

The straight part
We know that "Old copyists" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old copyists = scriveners.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 SCRIVENERS mean?
SCRIVENERS noun- A professional writer; one whose occupation is to draw contracts or prepare writings.
- One whose business is to place money at interest; a broker.
- A writing master.