Green’s Dictionary of Slang

clack v.

1. to fool by deceptive speech.

[UK]Ordinary of Newgate Account 8 Nov. 🌐 W – in his Livery, was often employ’d to clack the Carriers.

2. to chatter; to abuse.

implied in clack the doctor
[Ire]J. O’Keeffe Farmer 26: Dear Madam, how you clack away / [...] / Tuck up your Duds, and pack away.
‘The humours of Dunmow’ in Vocal Mag. 1 Apr. 124: Does any many in his enses wish to part with a good goose because shoo clacks a bit?
[UK]Yorks. Gaz. 6 Sept. 4/5: Mrs Husband will leave off clacking to Mrs Clack and Mr Clack will never again box the ears of Mrs Husband, clack as she may.
[UK]Egan Anecdotes of the Turf, the Chase etc. 25: (For since those who can’t fight always clack us).
[UK]Satirist (London) 4 Dec. 277/2: Parson Irving [...] has taken to a new trade. The unknown tongue has brought no grist to the mill—it goes ‘clack, clack,’ with nothing to grind.
[Aus]Truth (Sydney) 15 July 1/6: Roarings loud and pipings weak, / Rise from every clacking clique.
[UK]F. Leo [perf. Wilkie Bard] ‘The Barman’ 🎵 Oh my! talk about a chatter box / You ought to ’ear the old boy clack.

In phrases

clack the doctor (v.)

(UK Und.) to impersonate a doctor in order to rob a surgeon.

[UK] Ordinary of Newgate Accounts 8 Nov. 🌐 Fluellin dress’d up W– like a Countryman, in a riding Coat, girt round him with a broad sacking Belt, buckled with four Buckles, intencing to * clack the Doctor* [...] *Clacking the Doctor, is when one of the Gang personates a Physician, and under Pretence of coming with a Patient to the Surgeon for Advice, they watch their Opportunity to rob him.