Green’s Dictionary of Slang

crisp one n.

[the crisp texture of new notes]

a currency note; usu. £1.

[UK] ‘’Arry in ’Arrygate’ in Punch 24 Sept. 133/1: He tipped us a fifty-quid crisp ’un.
[Ire]P. McCabe Breakfast on Pluto 35: I shall relieve you of three hundred crisp ones as off to the city I swoosh.