Physicist in love with Her Majesty (3)
I believe the answer is:
ohm
'physicist' is the definition.
(Ohm is an example)
'love with her majesty' is the wordplay.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'with' is a charade indicator (letters next to each other).
'her majesty' becomes 'hm' ().
'o'+'hm'='OHM'
'in' is the link.
(Other definitions for ohm that I've seen before include "Physicist" , "SI unit of electrical resistance" , "measure of resistance" , "A little resistance" , "lawmaker" .)