The most common and most likely answer to this crossword clue is the 10 letter word PROMPTUARY
P
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Buddy explains!
The straight part
We know that "fact-book" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fact-book = promptuary.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 PROMPTUARY mean?
PROMPTUARY noun- A magazine, repository, or handbook.
- A device described in Napier's treatise Rabdologiæ, consisting of a large set of strips used to multiply multi-digit numbers more easily than Napier’s bones.
- Of or relating to preparation.