Queen protected by large resistance units (4)
I believe the answer is:
ohms
'resistance units' is the definition.
(ohm is a kind of resistance unit)
'queen protected by large' is the wordplay.
'queen' becomes 'hm'.
'protected by' is an insertion indicator.
'large' becomes 'os'.
'hm' going within 'os' is 'OHMS'.
(Other definitions for ohms that I've seen before include "Frank marking" , "measures of resistance" , "degrees of resistance" , "Letters with official status in correspondence" , "Electricity" .)