Saw guide that might be found in bishop's wardrobe? (5,3)

I believe the answer is:

mitre box

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'saw guide' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for mitre box that I've seen before include "providing help with joint construction" , "piece of bishop's luggage?" , "Case of bishop, perhaps" , "Saw guide" .)

I've seen this clue in The Guardian.

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