Card one's not taken seriously (5)
I believe the answer is:
joker
'card' is the definition.
(joker is a type of playing card)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for joker that I've seen before include "may appear wild in play" , "Buffoon" , "Comic; a card" , "trickster" , "Pack's extra card" .)