Old queen in charge of resistance unit (5)
I believe the answer is:
ohmic
'of resistance unit' is the definition.
(relating to ohms, units of electrical resistance)
'old queen in charge' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'queen' becomes 'HM' (short for Her Majesty).
'in charge' becomes 'ic' (abbreviation for 'in charge').
'o'+'hm'+'ic'='OHMIC'
(Other definitions for ohmic that I've seen before include "concerned with resistance" , "concerning resistance unit" , "Relating to electrical resistance" .)