Best racer has no limits (3)
I believe the answer is:
ace
'best' is the definition.
(ace can informally mean very good)
'racer has no limits' is the wordplay.
'has no limits' indicates the central letters.
The centre of 'racer' is 'ACE'.
(Other definitions for ace that I've seen before include "reason for point of no return" , "Best type of tennis serve" , "Top card or winning tennis serve" , "Tennis serve that the receiver can't reach" , "Entertain" .)