One setting off fireworks -- maybe Stephenson? (6,8)
I believe the answer is:
rocket launcher
I believe this clue is a double definition.
'one setting off fireworks' is the first definition.
(something launching rockets or missiles)
'maybe stephenson?' is the second definition.
(Robert Stephenson, inventor of Stephenson's Rocket)
(Other definitions for rocket launcher that I've seen before include "bazooka" , "weapon" .)