Sleeve you put your arm into (7)

I believe the answer is:

holster

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

'sleeve' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe there's an association between them I don't understand?

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for holster that I've seen before include "Revolver' cover" , "It can carry 17 Ac" , "Gun-holder" , "Leather case" , "Pistol sheath" .)

I've seen this clue in the Evening Standard.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.