Sweet nothing in the sack (7)
I believe the answer is:
beloved
'sweet' is the definition.
(thesaurus)
'nothing in the sack' is the wordplay.
'nothing' becomes 'love' (love is zero in tennis).
'in' is an insertion indicator.
'the sack' becomes 'bed' (slang as in 'in the sack').
'love' placed within 'bed' is 'BELOVED'.
(Other definitions for beloved that I've seen before include "Dearest, darling" , "Very dear" , "Much adored person" .)