Champion runs out of speed (3)
I believe the answer is:
ace
'champion' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for ace that I've seen before include "Adept" , "Top card or champion" , "One in playing-cards" , "Tennis serve that the opponent can't reach" , "Playing card with a single symbol" .)