Joint leader in church may let it go to his head (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 can't explain the remainder of the clue.

(Other definitions for mitre that I've seen before include "Merit (anag.)" , "in which one might find a bishop" , "Bishop's cap - wood joint" , "Corner joint - bishop's hat" , "bishop's head?" .)

I've seen this clue in The Times.

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