The most common and most likely answer to this crossword clue is the 5 plus 5 letter word MAGNA CARTA
I've seen this crossword clue in The Guardian
M
A
G
N
A
C
A
R
T
A
More clues leading to the result MAGNA CARTA
- Periodical not available to convey a historic document
- Catamaran ordered to transport grand historic document
- ‘Cat’ anagramis ‘act’?
- Historic document promoted cause of abstract artist to preserve school
- Poor anagram at start to clue historic document
- Agreement commemorated in 2015 revised as act
- John had to sign it after Armagnac drunk, thanks
- Reform Actfor historic document
- Reform Act — anagram for historic document
- Periodical rejected article about illustrations on a medieval manuscript
Buddy explains!
The straight part
We know that "Old document" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old document = magna carta.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.