Start playing tennis to be of use (5)
I believe the answer is:
serve
I believe this clue is a double definition.
'start playing tennis' is the first definition.
(I've seen this in another clue)
'be of use' is the second definition.
(I've seen this in another clue)
'to' is the link.
(Other definitions for serve that I've seen before include "Wait at table" , "Be of use to - wait on" , "commence action in court before judges?" , "get game started" , "Act as waiter" .)