Heavy Metal single covered by Big Mama (6)
I believe the answer is:
osmium
'heavy metal' is the definition.
(I've seen this before)
'single covered by big mama' is the wordplay.
'single' becomes 'i' (one).
'covered by' indicates putting letters inside.
'big' becomes 'os'.
'mama' becomes 'mum' ('mum' can be a synonym of 'mama').
'os'+'mum'='osmum'
'i' placed into 'osmum' is 'OSMIUM'.
(Other definitions for osmium that I've seen before include "Metal, the densest natural element (symbol Os)" , "Densest of all naturally occurring metallic elements, Os" , "Heavy metal" , "Heaviest substance known" , "Lustrous, extremely hard member of the platinum metals group" .)