I’m interrupting musician that’s rejected dense material (6)
I believe the answer is:
osmium
'dense material' is the definition.
(osmium is a dense metal)
'i'm interrupting musician that's rejected' is the wordplay.
'interrupting' indicates putting letters inside.
'musician' becomes 'muso' (muso can informally mean a musician).
'that's rejected' shows that the letters should be reversed in order.
'im' going within 'muso' is 'muimso'.
'muimso' written backwards gives 'OSMIUM'.
(Other definitions for osmium that I've seen before include "Metallic element" , "Densest of all naturally occurring metallic elements, Os" , "Chemical element" , "Lustrous, extremely hard member of the platinum metals group" , "The heaviest metal known" .)