Cheat getting round a new rule (5)
I believe the answer is:
'rule' is the definition.
(canon can mean a law)
'cheat getting round a new' is the wordplay.
'cheat' becomes 'con' (con is a kind of cheat).
'getting round' is an insertion indicator.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'con' placed around 'an' is 'CANON'.
(Other definitions for canon that I've seen before include "Principle; list of recognised works" , "priest for cathedral duties" , "Such literary works are genuine" , "Collection of writing" , "Body of works" .)