Cover for bishop's joint (5)

I believe the answer is:

mitre

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'joint' is the definition.
(I know that mitre is a type of joint)

I don't understand the rest of the clue.

(Other definitions for mitre that I've seen before include "Merit (anag.)" , "Remit (anag.)" , "Corner joint - bishop's hat" , "Kind of joint" , "Timer (anag.)" .)

I've seen this clue in The Independent.

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