Material possibly dangerous when used by top map-makers (8)

I believe the answer is:

asbestos

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'material' is the definition.
(asbestos is a kind of material)

'when used by top map-makers' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'used by' is a charade indicator (letters next to each other).
'top' becomes 'best' (similar in meaning).
'map-makers' becomes 'os' (Ordnance Survey).
'as'+'best'+'os'='ASBESTOS'

'possibly dangerous' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.

Can you help me to learn more?

(Other definitions for asbestos that I've seen before include "fire protection" , "Fire defence" , "Fireproof material, now known to be dangerous" , "Fire-retardant material" , "retardant" .)

I've seen this clue in The Times.

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