To CLENCH,
to nick a Business by timing it; as The Cull has clench'd the Job at a Pinch. The Rogue has nick'd the Time, before any Passengers came by, who might have rescu'd the plunder'd Person.
The Vulgar Tongue originally collected by Grose, 1811.
Also see examples in Canting Songs and Slang Rhymes collected by John S. Farmer (1896).