Black stuff with core of soft or hard material (5)
I believe the answer is:
Here is my best explanation:
'hard material' is the definition.
'black stuff with core of soft' is the subsidiary indication.
'black stuff' becomes 'coal'.
'with core of' means one lot of letters goes inside another.
'soft' becomes 'p' (musical abbreviation for 'piano').
'coal' going around 'p' is 'copal'.
'or' is the link.
(Other definitions for copal that I've seen before include "Aromatic tree resin", "Hard resin" and "Resin used in varnish".)