Rule 1 when opening arms (9)
I believe the answer is:
ordinance
'rule' is the definition.
(ordinance can mean an order or rule)
'1 when opening arms' is the wordplay.
'1' becomes 'I' (Roman numeral).
'when opening' means one lot of letters goes inside another (some letters open a gap in others).
'arms' becomes 'ordnance' (ordnance means weapons).
'i' inserted into 'ordnance' is 'ORDINANCE'.
(Other definitions for ordinance that I've seen before include "Authoritative decree" , "Canon" , "law" , "planning" , "Regulation" .)