The Queen and her son-in-law rolling a joint (5)

I believe the answer is:

mitre

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'a joint' is the definition.
(mitre joint is a kind of joint)

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

(Other definitions for mitre that I've seen before include "that goes over a cleric's head?" , "Sort of joint" , "A bishop wears it" , "See boss gear up" , "Merit (anag.)" .)

I've seen this clue in The Telegraph.

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