Heavy metal band starts to steal Motörhead's iconography with a couple of umlauts (6)

I believe the answer is:

osmium

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'heavy metal' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for osmium that I've seen before include "Metal, the densest natural element (symbol Os)" , "The heaviest metal known" , "Heaviest substance known" , "Lustrous, extremely hard member of the platinum metals group" , "Metallic element" .)

I've seen this clue in The Independent.

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