The most common and most likely answer to this crossword clue is the 10 letter word MICROFICHE
I've seen this crossword clue in The Guardian
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Buddy explains!
The straight part
We know that "archive medium" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case archive medium = microfiche.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 MICROFICHE mean?
MICROFICHE noun- A sheet of microfilm, six by four inches, holding several hundred reduced images of document pages; read using a microfiche reader or microfilm reader.
- A device used to magnify and read these sheets.