The most common and most likely answer to this crossword clue is the 13 letter word BLUNDERBUSSES
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Buddy explains!

The straight part
We know that "outdated arms" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case outdated arms = blunderbusses.The cryptic/wordplay part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 BLUNDERBUSSES mean?
BLUNDERBUSSES noun- An old style of muzzleloading firearm and early form of shotgun with a distinctive short, large caliber barrel that is flared at the muzzle, therefore able to fire scattered quantities of nails, stones, shot, etc. at short range.