Nothing right, darling (5)
I believe the answer is:
lover
'darling' is the definition.
(thesaurus)
'nothing right' is the wordplay.
'nothing' becomes 'love' (love is zero in tennis).
'right' becomes 'r' (common abbreviation).
'love'+'r'='LOVER'
(Other definitions for lover that I've seen before include "He really likes" , "I could be fickle" , "Beau" , "Devotee" , "'Lady Chatterley's - - ' by D H Lawrence" .)