Unit giving nothing to Her Majesty (3)
I believe the answer is:
ohm
'unit' is the definition.
(ohm is a kind of unit)
'giving nothing to her majesty' is the wordplay.
'giving' means one lot of letters go next to another.
'nothing' becomes 'O' (O resembles 0).
'to her majesty' becomes 'hm' (short for Her Majesty).
'o' put next to 'hm' is 'OHM'.
(Other definitions for ohm that I've seen before include "A little resistance" , "physical unit" , "measure of resistance" , "SI unit of electrical resistance" , "Physicist" .)