Concubine loses son three times in joint (5)

I believe the answer is:

mitre

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

I can't explain the remainder of the clue.

(Other definitions for mitre that I've seen before include "Timer for a bishop." , "Timer (anag.)" , "Episcopal headdress" , "in which one might find a bishop" , "Bishop's headwear" .)

I've seen this clue in The Telegraph.

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