Lowest card in a royal flush (3)
I believe the answer is:
ten
'lowest card in a royal flush' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ten that I've seen before include "No of sides of a decagon" , "Dudley Moore/Bo Derek film" , "Cardinal number" , "honour" , "Roman numeral X" .)