Low card in a royal flush (3)
I believe the answer is:
ten
'low card in a royal flush' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ten that I've seen before include "PM's address, Downing St" , "time" , "How many" , "Five times two" , "Dudley Moore/Bo Derek film" .)