Nothing but affection (4)
I believe the answer is:
love
This clue is a double definition.
'nothing' is the first definition.
(love means zero in tennis)
'affection' is the second definition.
(I know that affection can be written as love)
'but' is the link.
(Other definitions for love that I've seen before include "Strong affection" , "Disappointing score" , "Zero (tennis) four Dn" , "Nil score in tennis, dear" , ""Tell me the truth about ___" (Auden)" .)