Competitor cheated dreadfully, having got let in (10)

I believe the answer is:

decathlete

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'competitor' is the definition.
(I have seen 'versatile competitor' mean 'decathlete' so perhaps 'competitor' could also mean 'decathlete')

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for decathlete that I've seen before include "versatile competitor" , "sportsman" , "All-rounder" .)

I've seen this clue in The Telegraph.

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