Rifle butt might contain this (4)

I believe the answer is:

sack

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'rifle' is the definition.
(I've seen this before)

I don't understand how the rest of the clue works.

(Other definitions for sack that I've seen before include "Fire - old Spanish wine" , "Dismiss; plunder; wine" , "Plunder; wine; bag" , "Wine; fire; bag" , "Dismiss (from a job)" .)

I've seen this clue in The Guardian.

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