Os, the heaviest known metal (6)

I believe the answer is:

osmium

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

I've seen this clue in The Guardian.

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