Green’s Dictionary of Slang

card n.1

[sure card under card n.2 ]

a device, an expedient.

F. Brooke Hist. of Lady J. Mandeville 31: Poor fellow! I pity him; but marriage is his only card.
[UK]W. Scott Woodstock III 358: No card seemed to turn up favourable to the royal cause.
[UK]G.J. Whyte-Melville Digby Grand (1890) 231: [of a horse] ‘Polly’s the card for you to stick to, ain’t she, Grand?’.