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

I believe the answer is:
osmium
'heavy metal' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(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" .)
