Truce after weapon destroyed cities (9)
I believe the answer is:
armistice
'truce' is the definition.
('armistice' can be a synonym of 'truce')
'after weapon destroyed cities' is the wordplay.
'after' means one lot of letters go next to another.
'weapon' becomes 'arm' (I've seen this before).
'destroyed' indicates an anagram.
'cities' is an anagram of 'istice'.
'arm' next to 'istice' is 'ARMISTICE'.
(Other definitions for armistice that I've seen before include "Truce" , "End of hostilities" , "Cease-fire in order to conduct talks" , "Suspension of hostilities" , "harbinger of peace?" .)