Green’s Dictionary of Slang

tap n.³

(US) a robbery; a confidence trick.

1934		D. Hammett Thin Man 673: [S]he figured maybe he'd be worth more for a long pull than for a quick tap and a getaway, so she gave him the business and and wound up with a steady connection.
1950		Goldin et al. DAUL 220/1: Tap, the, n. [...] 2. A successful swindling operation involving a moderate sum of money; an extortion of a small amount of money.
1968		J. Colebrook Cross of Lassitude 101: The termites of the life [...] who carry out the ‘lemon,’ the ‘tap,’ the ‘wire.’.

In phrases

at the tap (phr.)

(UK und.) selling false information.

1935

Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks 1: At the tap: Selling false information.