One is found in chapter, as a rule (5)
I believe the answer is:
canon
'a rule' is the definition.
(canon is a kind of rule)
'one is found in chapter' is the wordplay.
'one' becomes 'an' (I've seen this in other clues).
'is found in' is an insertion indicator.
'chapter' becomes 'con' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'an' going inside 'con' is 'CANON'.
'as' is the link.
Can you help me to learn more?
(Other definitions for canon that I've seen before include "Such literary works are genuine" , "Member of a cathedral chapter" , "collected works" , "Collection of writing" , "Moron" .)