Chemical to use when short of metal? (4)

I believe the answer is:

alum

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'metal?' is the definition.
(alum is a kind of metal)

I can't explain the remainder of the clue.

(Other definitions for alum that I've seen before include "Sulphate used in dyeing and tanning" , "Name given to a specific chemical compound and a class of them" , "Compound used in tanning and dying" , "White mineral salt used in medicine and in dyeing" , "Aluminium/potassium sulphate" .)

I've seen this clue in the Evening Standard.

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