It's a crime to break a truce (9)
I believe the answer is:
armistice
'a truce' is the definition.
(I know that truce can be written as armistice)
'it's a crime to break' is the wordplay.
'to break' indicates an anagram.
'its'+'a'+'crime'='itsacrime'
'itsacrime' anagrammed gives 'ARMISTICE'.
(Other definitions for armistice that I've seen before include "Cease-fire in order to conduct talks" , "Agreement during war to suspend fighting" , "when shooting stops" , "Truce" , "Fire is prevented by this" .)