Case in point symbolises this valuable material (8)
I believe the answer is:
platinum
'this valuable material' is the definition.
'case in point symbolises' is the wordplay.
I cannot really understand how this works, but
'in' is within the answer.
This explanation may well be incorrect...
(Other definitions for platinum that I've seen before include "Precious metal, Pt" , "Heavy white metal" , "1 down, symbol Pt" , "Silver-white precious metal" , "Precious metal; sort of blonde" .)