Good card in blackjack (3)
I believe the answer is:
ace
'good card in blackjack' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ace that I've seen before include "fighter pilot" , "Card with a single pip" , "One-spotted card" , "Top card or winning tennis serve" , "SERVE?] that's non-returnable" .)