Decree not repeated about broken drain (9)
I believe the answer is:
ordinance
'decree' is the definition.
(I've seen this before)
'not repeated about broken drain' is the wordplay.
'not repeated' becomes 'once' (I am not sure about this - if you are sure you should believe this answer much more).
'about' means one lot of letters goes inside another.
'broken' indicates an anagram.
'drain' with letters rearranged gives 'rdina'.
'once' enclosing 'rdina' is 'ORDINANCE'.
Can you help me to learn more?
(Other definitions for ordinance that I've seen before include "planning" , "Edict, order" , "Bye-law of a city government" , "Command" , "Canon" .)