Crossword Genius

Card game wherein jack trumps first card (5)

Ross

I believe the answer is:

joker

'card' is the definition.
(thesaurus)

I don't understand the rest of the clue.

(Other definitions for joker that I've seen before include "53rd card in the pack?" , "Humorist" , "trickster" , "Card - wag" , "Jester" .)

I've seen this clue in The Times.
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