Jam's without sharpness (4)

I believe the answer is:

edge

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'sharpness' is the definition.
(I know that sharpness can be written as edge)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for edge that I've seen before include "Boundary; advantage" , "Bind" , "Move little by little to the border" , "Lip; advantage" , "Golden ingot" .)

I've seen this clue in The Guardian.

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