Units that show resistance working for the government (4)
I believe the answer is:
ohms
I believe this clue is a double definition.
'units that show resistance' is the first definition.
(the ohm is the unit of electrical resistance)
'working for the government' is the second definition.
(short for On Her Majesty's Service)
(Other definitions for ohms that I've seen before include "Units of electrical resistance" , "Frank marking" , "Electricity" , "Measure of resistance" , "degrees of resistance" .)