Forty all in tennis (5)
I believe the answer is:
deuce
'forty all in tennis' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for deuce that I've seen before include "40-40 in tennis -- 2 in cards" , "with equal score" , "score, forty apiece" , "Dickens" , "More than one" .)