Green’s Dictionary of Slang

1. a sharp blow.

1826		Pierce Egan’s Life in London 19 Mar. 477/2: [H]e received such a clinker near one of his ogles, that gave him the staggers.
c.1840		Thackeray Mr and Mrs. Frank Berry (1887) 259: Berry goes gallantly in, and delivers a clinker on the gown-boy’s jaw.
1880		Bulletin (Sydney) 13 Mar. 5/2: We would require a wager or a clinker to force this into our skulls.
1906		E. Dyson Fact’ry ’Ands 249: Er lad name iv Creegan [...] what got ther sudden jerk fer punchin’ ther boss er clinker in ther whiskers.
1916		G. Squiers Aerbut Paerks, of Baernegum 4: ’E copt the bloke a clinker in the neck with ’is left ’oof.

2. a shock, a suprise.

2020

L. Lungaro The 3-0 227: The captain let out a deep breath and hit me with the clinker. Like a sledge hammer to the head, it was as if I was Rocky Balboa getting hit by Apollo Creed.