For instance, bone comprises optimum insulating material (8)
I believe the answer is:
asbestos
'insulating material' is the definition.
(material formerly used as insulation)
'for instance bone comprises optimum' is the wordplay.
I cannot quite see how this works, but
'for' could be 'to' (eg both can mean 'in the opinion of') and 'to' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for asbestos that I've seen before include "I might end up lagging" , "Fibrous mineral, used as an insulator, now deemed harmful" , "retardant" , "Fire-resistant material dangerous to health" , "Once used for safety, it proved dangerous" .)