Continue to preserve law (5)
I believe the answer is:
canon
'law' is the definition.
(thesaurus)
'continue to preserve' is the wordplay.
'continue' becomes 'on' (I've seen this in other clues).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'preserve' becomes 'can' (canning is a kind of preserving).
'on' put after 'can' is 'CANON'.
(Other definitions for canon that I've seen before include "Principle - church dignitary - regulation" , "Senior clergyman" , "General rule; musical form" , "General principle or kind of clergyman" , "Body of generally established principles for a given context" .)