Metal having variable edge — not hard (4)

I believe the answer is:

zinc

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'metal' is the definition.
(zinc is a metal)

I cannot understand how the rest of the clue works.

(Other definitions for zinc that I've seen before include "coat for protection" , "Penultimate element (alpha. order)" , "Metal used in galvanising iron" , "Anti-corrosion agent" , "Ingredient in brass" .)

I've seen this clue in The Times.

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