Jack or joker (4)
I believe the answer is:
card
This clue is a double definition.
'jack' is the first definition.
(I've seen this before)
'joker' is the second definition.
(joker is a type of playing card)
(Other definitions for card that I've seen before include "Comb (wool)" , "Greetings ----" , "caution" , "Piece of stiff paper" , "appears in a suit" .)