Arsenal reserves warm up (4)

I believe the answer is:

guns

I'm a little stuck... Click here to teach me more about this clue!

'arsenal reserves' is the definition.
Although both the answer and definition are verbs in their -s form, I cannot understand how one could define the other.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for guns that I've seen before include "Firearms" , "Snug weapons?" , "Transportable armaments" , "Bangers" .)

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