Bouncer employed for rallies? (6,4)

I believe the answer is:

tennis ball

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'bouncer' is the definition.
(tennis balls bounce)

I cannot understand how the remainder of the clue works.

(Other definitions for tennis ball that I've seen before include "Wimbledon equipment" , "Something hit by racquets" , "Participant in rally" , "Item for Wimbledon game" , "Item hit with rackets" .)

I've seen this clue in The Times.

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