Jack, the rascal (5)
I believe the answer is:
This clue is a double definition.
'jack' is the first definition.
(knave can mean a jack playing card)
'the rascal' is the second definition.
(Other definitions for knave that I've seen before include "Scoundrel - card" , "cheat of old" , "Villain - jack" , "Rat" , "Jack - rogue" .)