Maintain preservation - that’s the rule (5)
I believe the answer is:
canon
'the rule' is the definition.
(canon can mean a law)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for canon that I've seen before include "Collection of writing" , "General rule; musical form" , "Big type" , "minister?" , "Ecclesiastical rule" .)