Dangerous material as used by top mapmakers (8)
I believe the answer is:
asbestos
'dangerous material' is the definition.
(I've seen this before)
'as used by top mapmakers' is the wordplay.
'used by' is a charade indicator (letters next to each other).
'top' becomes 'best' (associated in meaning).
'mapmakers' becomes 'OS' (Ordnance Survey).
'as'+'best'+'os'='ASBESTOS'
(Other definitions for asbestos that I've seen before include "Fire-resistant material dangerous to health" , "we now know to be dangerous" , "Incombustible material" , "Fibrous material, doesn't burn" , "you can't inflame this" .)