It's a crime breaking peace treaty (9)
I believe the answer is:
armistice
'peace treaty' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'it's a crime breaking' is the wordplay.
'breaking' indicates anagramming the letters.
'its'+'a'+'crime'='itsacrime'
'itsacrime' with letters rearranged gives 'ARMISTICE'.
Can you help me to learn more?
(Other definitions for armistice that I've seen before include "Cease-fire in order to conduct talks" , "Fire is prevented by this" , "let-up from war" , "Agreement during war to suspend fighting" , "harbinger of peace?" .)