It's nothing, sweetheart (4)
I believe the answer is:
love
I believe this is a double definition.
'it's nothing' is the first definition.
(love is zero in tennis)
'sweetheart' is the second definition.
(both are terms of address to romantic partner)
(Other definitions for love that I've seen before include "Serenade" , "Passion" , "Vole (anag.)" , "Nil score in tennis, dear" , "See 10" .)