A clergyman as a rule (5)
I believe the answer is:
canon
I believe this clue is a double definition.
'a clergyman' is the first definition.
(canon is a kind of clergyman)
'a rule' is the second definition.
(canon can mean a law)
'as' acts as a link.
(Other definitions for canon that I've seen before include "Member of a cathedral chapter" , "Senior clergyman" , "Round, general rule" , "Collection of writing" , "Literary works of quality" .)