Highest card in a royal flush (3)
I believe the answer is:
ace
'highest card in a royal flush' 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 "Tennis serve that the receiver can't reach" , "Crack" , "SERVE?] that's non-returnable" , "Brill" , "smashing" .)