I leave the law to the big guns (8)
I believe the answer is:
ordnance
'guns' is the definition.
(ordnance can mean guns or artillery)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for ordnance that I've seen before include "Mounted guns, cannon" , "Car on end (anag) - artillery" , "regulation" , "Cannon; munitions" , "guns etc." .)