Characters associated with government business in resistance units (4)
I believe the answer is:
ohms
I believe this clue is a double definition.
'characters associated with government business' is the first definition.
(abbreviation for On Her Majesty's Service)
'resistance units' is the second definition.
(ohm is the unit of electrical resistance)
'in' acts as a link.
(Other definitions for ohms that I've seen before include "Units of electrical resistance" , "Letters with official status in correspondence" , "working for the monarchy?" , "Frank marking" , "Measure of resistance" .)