Order unit to intercept arms (9)
I believe the answer is:
ordinance
'order' is the definition.
(ordinance can mean an order or edict)
'unit to intercept arms' is the wordplay.
'unit' becomes 'I' (resembles 1, a unit).
'to intercept' is an insertion indicator.
'arms' becomes 'ordnance' (ordnance means weapons).
'i' placed within 'ordnance' is 'ORDINANCE'.
(Other definitions for ordinance that I've seen before include "Regulation" , "Showing resolution" , "Edict, order" , "religious service maybe" , "Authoritative decree" .)