Possibly gun concealed by king in Christmas cap (8)

I believe the answer is:

yarmulke

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'cap' is the definition.
(yarmulke is a kind of cap)

I cannot understand how the rest of the clue works.

(Other definitions for yarmulke that I've seen before include "Jewish skullcap; leaky rum (anag.)" , "A skullcap worn by Jewish men" , "Skullcap worn by Jewish man" , "A Jewish man's skullcap" , "Jewish headgear" .)

I've seen this clue in The Telegraph.

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