High card in a royal flush (3)
I believe the answer is:
ace
'high 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 "1" , "Player who excels" , "In golf, a hole in one" , "Tennis serve that the receiver is unable to reach" , "Unbeatable service" .)