Green’s Dictionary of Slang

leading card n.

[card-playing jargon; the card that is led sets the initial betting standard for a round of play]

an example or precedent.

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[UK]B.E. Dict. Canting Crew.
[UK]F.L.G. Swells Night Out n.p.: She has been honoured with the protection of one of the first noblemen in the land, and is a ‘leading card’ in every place of public entertainment [...] beauty like hers is not to be bought for a trifle.