Card in a blackjack (3)
I believe the answer is:
ace
'card in a blackjack' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ace that I've seen before include "One in playing-cards" , "The opponent can't return this serve" , "Player who excels" , "Best type of tennis serve" , "One-spotted card" .)