Metallic element split in two somehow (6)

I believe the answer is:

osmium

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'metallic element' is the definition.
(osmium is a kind of metallic element)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for osmium that I've seen before include "Heaviest substance known" , "Metal, the densest natural element (symbol Os)" , "Chemical element" , "The heaviest metal known" , "Silvery-white metal, Os" .)

I've seen this clue in the Independent.

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