Puts the ball in play on court (6)
I believe the answer is:
serves
'puts the ball in play on court' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for serves that I've seen before include "Strokes that puts the ball in play" , "What a waiter does" , "Puts the tennis ball in play" , "Puts the ball in play on court" .)