Some hope at first for English breaking such a law (4)
I believe the answer is:
ohms
'such a law' is the definition.
(as in Ohm's law in electronics)
I can't explain the remainder of the clue.
(Other definitions for ohms that I've seen before include "Frank marking" , "measures of resistance" , "Frank on official post" , "Electricity" , "working for the monarchy?" .)