US car notably breaking the law (12)
I believe the answer is:
constabulary
'the law' is the definition.
('constabulary' can be a synonym of 'law')
'us car notably breaking' is the wordplay.
'breaking' indicates an anagram.
'us'+'car'+'notably'='uscarnotably'
'uscarnotably' anagrammed gives 'CONSTABULARY'.
(Other definitions for constabulary that I've seen before include "Police force or station" , "14 6 or 2" .)