Explosive switched for one type of coal (9)
I believe the answer is:
gelignite
'explosive' is the definition.
(gelatin-based explosive substance)
'switched for one type of coal' is the wordplay.
'switched' says the letters should be written in reverse.
'for one' becomes 'eg' (both can mean 'for example').
'type of coal' becomes 'lignite' (I've seen this in another clue).
'eg' backwards is 'ge'.
'ge'+'lignite'='GELIGNITE'
(Other definitions for gelignite that I've seen before include "factor in altering mine?" , "Tieing leg up in explosive" , "source of flare-up" , "(E.g. quarry) explosive" , "Explosive used in mining" .)