Broadcast a race (5)

I believe the answer is:

relay

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'broadcast' is the definition.
(relay can mean to broadcast or pass on)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for relay that I've seen before include "Hand, pass on" , "track event" , "Pass on - put down again" , "Broadcast" , "supplementary store of anything" .)

I've seen this clue in The Mirror.

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