40-40, in tennis (5)
I believe the answer is:
deuce
'40-40 in tennis' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for deuce that I've seen before include "A two; forty all" , "Two in cards or dice" , "situation of no advantage to either side" , "40-40 in tennis -- 2 in cards" , "Level of tennis" .)