Contest in which every competitor's bagged first (4,4)

I believe the answer is:

sack race

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'contest' is the definition.
(sack race is a kind of contest)

I can't explain the rest of the clue.

(Other definitions for sack race that I've seen before include "'Novelty competition where entrants run with their feet in a bag (4,4)'" , "Get rid of nation" , "event - with 10d" , "Novelty competition on school sports day" , "Jumping contest" .)

I've seen this clue in The Telegraph.

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