Jack's a card (5)
I believe the answer is:
knave
I believe this clue is a double definition.
'jack's' is the first definition.
(knave can mean a jack playing card)
'a card' is the second definition.
(I know that knave is a type of court card)
(Other definitions for knave that I've seen before include "Villain - card" , "Blackguard" , "pack member?" , "Jack - rogue" , "Rogue - card" .)