Cutting into beam it requires 45 degree angle (5)
I believe the answer is:
mitre
'angle' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for mitre that I've seen before include "A bishop wears it" , "Liturgical headdress" , "that goes over a cleric's head?" , "Bishop's headgear - joint" , "top?" .)