Gun has removed third law (5)
I believe the answer is:
canon
'law' is the definition.
(I've seen this in another clue)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for canon that I've seen before include "Member of a cathedral chapter" , "General principle or senior priest" , "Part of chapter" , "collected works" , "Church decree or law" .)