Zero, in tennis (4)
I believe the answer is:
love
'zero in tennis' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for love that I've seen before include "Feel great affection for" , "0 (tennis)" , "See 10" , "No points" , "1970 film, starring Ali MacGraw and Ryan O'Neal" .)