Flexible card in blackjack (3)
I believe the answer is:
ace
'flexible 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 "golfer's achievement" , "winner in court" , "Expert - winning serve" , "Person who excels, a top card" , "rare driving result?" .)