A law-breaking instrument (3)
I believe the answer is:
awl
'instrument' is the definition.
(I have seen 'Pointed instrument ' mean 'awl' so perhaps 'instrument' could also mean 'awl')
'a law-breaking' is the wordplay.
'breaking' is an anagram indicator.
'law' with letters rearranged gives 'AWL'.
(Other definitions for awl that I've seen before include "It bores" , "Shoemaker's tool" , "Tool for punching small holes" , "Tool for piercing holes in leather" , "Something needed for boring jobs" .)