Insulating material thrown at bosses (8)
I believe the answer is:
asbestos
'insulating material' is the definition.
(material formerly used as insulation)
'thrown at bosses' is the wordplay.
'thrown' indicates an anagram.
'at'+'bosses'='atbosses'
'atbosses' is an anagram of 'ASBESTOS'.
(Other definitions for asbestos that I've seen before include "Boss set a non-burning material up" , "Fire-resistant material dangerous to health" , "Fire-resistant mineral" , "Insulating mineral" , "Fire-resistant material, injurious to health" .)