The most common and most likely answer to this crossword clue is the 10 letter word PARCHMENTS
I've seen this crossword clue in Financial Times
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The straight part
We know that "Old manuscripts" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old manuscripts = parchments.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 PARCHMENTS mean?
PARCHMENTS noun- Material, made from the polished skin of a calf, sheep, goat or other animal, used like paper for writing.
- A document made on such material.
- A diploma (traditionally written on parchment).
- Stiff paper imitating that material.
- The creamy to tanned color of parchment.
- The envelope of the coffee grains, inside the pulp.