Card, but not king — knave (3)
I believe the answer is:
cad
'knave' is the definition.
(I've seen this before)
'card but not king' is the wordplay.
'but not' indicates named letters should be taken away.
'king' becomes 'r' (r is abbreviation for rex).
'card' with 'r' removed is 'CAD'.
(Other definitions for cad that I've seen before include "Rotter, bounder" , "Stinker" , "A knave, real rotter" , "A rotter, by Jove" , "Wretch" .)