Fireproof top carried by online fashion retailer (8)
I believe the answer is:
asbestos
'fireproof' is the definition.
(fireproof material)
'top carried by online fashion retailer' is the wordplay.
'top' becomes 'best' (associated in meaning).
'carried by' means one lot of letters goes inside another.
'online fashion retailer' becomes 'ASOS' (online fashion company).
'best' put within 'asos' is 'ASBESTOS'.
(Other definitions for asbestos that I've seen before include "Heat-resistant material" , "Once used for safety, it proved dangerous" , "Incombustible material" , "Boss set a non-burning material up" , "Insulating mineral" .)