Crossword Genius

Rifle butt might contain this (4)

Ross

I believe the answer is:

sack

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

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

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

Can you help me to learn more?

(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.
Want a hint initially instead of a full solution? Install my app