Waits on (6)
I believe the answer is:
serves
'waits on' is the definition.
(to wait on someone is to serve them)
This is the entire clue.
(Other definitions for serves that I've seen before include "Puts the ball in play on court" , "What a waiter does" , "Strokes that puts the ball in play" , "Puts the tennis ball in play" .)