Blackjack card with two values
I believe the answer is:
ace
'blackjack card with two values' 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 "Outstanding" , "Tennis serve that the receiver can't reach" , "Playing card with single spot" , "fighter pilot" , "Tennis serve that the opponent can't reach" .)