Champion in speed contest losing lead (3)
I believe the answer is:
ace
'champion' is the definition.
(both can mean a person excelling at something)
'speed contest losing lead' is the wordplay.
'speed contest' becomes 'race' (I've seen this in another clue).
'losing lead' means to remove the first letter.
'race' with its initial letter taken away is 'ACE'.
'in' acts as a link.
(Other definitions for ace that I've seen before include "Top card or champion" , "Hole in one; card" , "Possibly a diamond" , "Tennis serve that the opponent can't reach" , "Best type of tennis serve" .)