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've seen this before)
'but' acts as a 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)" .)