As decreed, I go in the artillery (9)
I believe the answer is:
ordinance
'as decreed i' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'go in the artillery' is the wordplay.
'go' becomes 'i' (I can't explain this - if you can you should believe this answer much more).
'in the' means one lot of letters goes inside another.
'artillery' becomes 'ordnance' (synonyms).
'i' going into 'ordnance' is 'ORDINANCE'.
Can you help me to learn more?
(Other definitions for ordinance that I've seen before include "Bye-law of a city government" , "Bill" , "Rule" , "Showing resolution" , "law" .)