It's a crime breaking a truce (9)
I believe the answer is:
armistice
'a truce' is the definition.
('armistice' can be a synonym of 'truce')
'it's a crime breaking' is the wordplay.
'breaking' is an anagram indicator.
'its'+'a'+'crime'='itsacrime'
'itsacrime' is an anagram of 'ARMISTICE'.
(Other definitions for armistice that I've seen before include "Truce" , "Cease-fire in order to conduct talks" , "Agreement during war to suspend fighting" , "Suspension of hostilities" , "harbinger of peace?" .)