Fireproof material American bosses ordered? About time! (8)
I believe the answer is:
asbestos
'fireproof material' is the definition.
(I've seen this in another clue)
'american bosses ordered? about time' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'ordered?' indicates an anagram.
'about' means one lot of letters goes inside another.
'time' becomes 't' (abbreviation).
'bosses' with letters rearranged gives 'sbesos'.
'sbesos' placed around 't' is 'sbestos'.
'a'+'sbestos'='ASBESTOS'
(Other definitions for asbestos that I've seen before include "Heat-resistant fibrous mineral" , "retardant" , "Fireproof material, dangerous to health" , "Fire-resistant mineral" , "Survivor of heat" .)