Explosive a posh girl put a match to (9)
I believe the answer is:
gelignite
'explosive' is the definition.
(gelatin-based explosive substance)
'a posh girl put a match to' is the wordplay.
'a posh girl' becomes 'gel' (I've seen this in other clues).
'put a match to' becomes 'ignite' (I've seen this before).
'gel'+'ignite'='GELIGNITE'
(Other definitions for gelignite that I've seen before include "Explosive used in mining" , "source of flare-up" , "Tieing leg up in explosive" , "High explosive" , "factor in altering mine?" .)