Launch a tennis ball (5)
I believe the answer is:
serve
'launch a tennis ball' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for serve that I've seen before include "get game started" , "Milton 'They also . . . . . who only stand and wait'" , "Start (tennis game)" , "Deliver" , "Be of use to - wait on" .)