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Competitor
in
event
ran
tirelessly
(7)
Competitor in event ran tirelessly (7)
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The definition is at the beginning of the clue.
How the clue works
Right, so Competitor at the start is your definition — that's just someone taking part in an event. Now look inside "evENT RAN Tirelessly" — the answer's hiding there in plain sight, running through consecutive letters across those words to spell out ENTRANT.