Set to spark off explosive material (9)
I believe the answer is:
gelignite
'explosive material' is the definition.
(I know that Gelignite is a explosive material)
'set to spark' is the wordplay.
'set' becomes 'gel' (both can mean to harden).
'to spark' becomes 'ignite' (I've seen this in another clue).
'gel'+'ignite'='GELIGNITE'
'off' is the link.
(Other definitions for gelignite that I've seen before include "(E.g. quarry) explosive" , "Tieing leg up in explosive" , "factor in altering mine?" , "source of flare-up" , "High explosive" .)