One in the artillery as ordered (9)
I believe the answer is:
ordinance
'one in the artillery as ordered' is the definition.
I can't tell whether this definition defines the answer.
'one in the artillery' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in the' means one lot of letters goes inside another.
'artillery' becomes 'ordnance' (synonyms).
'i' placed into 'ordnance' is 'ORDINANCE'.
Can you help me to learn more?
(Other definitions for ordinance that I've seen before include "Canon" , "planning" , "religious service maybe" , "practice" , "Edict, order" .)