Cheat getting round a new rule (5)
I believe the answer is:
canon
'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).
'a'+'n'='an'
'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" .)