German physicist in love with His Majesty (3)
I believe the answer is:
ohm
'german physicist' is the definition.
(I've seen this in another clue)
'love with his majesty' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'with' means one lot of letters go next to another.
'his majesty' becomes 'hm' (short for His Majesty).
'o'+'hm'='OHM'
'in' acts as a link.
(Other definitions for ohm that I've seen before include "physical unit" , "lawmaker" , "measure of resistance" , "A little resistance" , "Physicist" .)