Explosive used by rude GP now, possibly (9)
I believe the answer is:
gunpowder
'explosive' is the definition.
(I know that gunpowder is a type of explosive)
'rude gp now possibly' is the wordplay.
'possibly' indicates an anagram.
'rude'+'gp'+'now'='rudegpnow'
'rudegpnow' with letters rearranged gives 'GUNPOWDER'.
'used by' is the link.
(Other definitions for gunpowder that I've seen before include "plot? Sort of" , "It's used in gunnery and fireworks" , "Explosive invented in 9th-century China" , "Explosive mixture (of tea?)" , "this plot failed" .)