Competitor entering event ran tiredly (7)
I believe the answer is:
entrant
'competitor' is the definition.
(someone who has entered a competition)
'entering event ran tiredly' is the wordplay.
'entering' indicates a hidden word.
'ENTRANT' can be found hidden inside 'event ran tiredly'.
(Other definitions for entrant that I've seen before include "Contender" , "Candidate" , "Contestant; starter" , "New participant" , "One going in for something" .)