Report of gun law (5)
I believe the answer is:
canon
'law' is the definition.
(thesaurus)
'report of gun' is the wordplay.
'report of' indicates a 'sounds like' (homophone) clue.
'gun' becomes 'cannon' (cannon is a kind of gun).
'cannon' is a homophone of 'CANON'.
(Other definitions for canon that I've seen before include "General rule; cleric" , "religious leader" , "Principle - member of cathedral staff" , "accepted criterion" , "Body of generally established principles for a given context" .)