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 wordplay.
'black stuff' becomes 'coal'.
'with core of' is an insertion indicator.
'soft' becomes 'p' (musical abbreviation for 'piano').
'coal' going around 'p' is 'COPAL'.
'or' is the link.