A race of carriers (5)

I believe the answer is:

relay

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'a race' is the definition.
(relay race)

I can't explain the rest of the clue.

(Other definitions for relay that I've seen before include "supplementary store of anything" , "Receive and pass on - race" , "Hand, pass on" , "Pass on - put down again" , "article in bank" .)

I've seen this clue in the Evening Standard.

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