Knave takes king for joker (4)
I believe the answer is:
card
'joker' is the definition.
(joker is a type of playing card)
'knave takes king' is the wordplay.
'knave' becomes 'cad' (I've seen this before).
'takes' is an insertion indicator.
'king' becomes 'r' (r is abbreviation for rex).
'cad' going around 'r' is 'CARD'.
'for' is the link.
(Other definitions for card that I've seen before include "king or queen, for example" , "caution" , "A jack, for example" , "Eg, a heart or club" , "Christmas greeting" .)