British bishop's headdress (5)
I believe the answer is:
mitre
'british bishop's headdress' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mitre that I've seen before include "Corner joint - bishop's hat" , "Bishop's cap - wood joint" , "Bishop's headwear" , "Bishop's headgear - joint" , "that goes over a cleric's head?" .)