Jack is scoundrel (5)
I believe the answer is:
knave
I believe this clue is a double definition.
'jack' is the first definition.
(knave can mean a jack playing card)
'is scoundrel' is the second definition.
(I've seen this in another clue)
(Other definitions for knave that I've seen before include "Rapscallion" , "Jack (card); base fellow" , "Jack - the cad!" , "[COURT CARD] , maybe" , "geezer" .)