Units formerly on government service (4)
I believe the answer is:
ohms
'units' is the definition.
(ohm is a kind of unit)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for ohms that I've seen before include "Units of electrical resistance" , "Measure of resistance" , "Letters with official status in correspondence" , "working for the monarchy?" , "Electricity" .)