Supply weapons before cities arranged a truce (9)
I believe the answer is:
armistice
'a truce' is the definition.
('armistice' can be a synonym of 'truce')
'supply weapons before cities arranged' is the wordplay.
'supply weapons' becomes 'arm' (I've seen this before).
'before' says to put letters next to each other.
'arranged' indicates anagramming the letters.
'cities' is an anagram of 'istice'.
'arm'+'istice'='ARMISTICE'
(Other definitions for armistice that I've seen before include "Truce" , "let-up from war" , "Suspension of hostilities" , "Cease-fire in order to conduct talks" , "Agreement during war to suspend fighting" .)