Heavy metal backing musician covering Iron Maiden's originals (6)
I believe the answer is:
osmium
'heavy metal' is the definition.
(I've seen this before)
'backing musician covering iron maiden's originals' is the wordplay.
'backing' says the letters should be written backwards.
'musician' becomes 'muso' (muso can informally mean a musician).
'covering' is an insertion indicator.
'originals' indicates taking the first letters (I've seen 'original' mean this).
The initial letters of 'iron maidens' is 'im'.
'muso' enclosing 'im' is 'muimso'.
'muimso' reversed gives 'OSMIUM'.
(Other definitions for osmium that I've seen before include "Heavy metal" , "Chemical element" , "Heaviest substance known" , "The heaviest metal known" , "Metal, the densest natural element (symbol Os)" .)