Jack makes them apt to fold sharply! (6)

I believe the answer is:

knives

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'jack makes' is the definition.
Although both the answer and definition are plural nouns, I cannot see how one could define the other.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for knives that I've seen before include "Cutters" , "Blades" , "Weapons" , "Cutting implements" , "Daggers" .)

I've seen this clue in the Evening Standard.

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