Jack, say: knave about right! (4)
I believe the answer is:
card
'jack say' is the definition.
(jack is a type of playing card)
'knave about right' is the wordplay.
'knave' becomes 'cad' (I've seen this before).
'about' is an insertion indicator.
'right' becomes 'r' (common abbreviation).
'cad' placed around 'r' is 'CARD'.
(Other definitions for card that I've seen before include "One taken in hand" , "eccentric character" , "Christmas greeting" , "A eccentric - a joker, perhaps" , "Posted greeting" .)