Projectile — backfire (9)
I believe the answer is:
boomerang
I believe this is a double definition.
'projectile' is the first definition.
(I know that boomerang is a type of projectile)
'backfire' is the second definition.
(I know that backfire can be written as boomerang)
(Other definitions for boomerang that I've seen before include "Aboriginal throwing stick" , "Primitive weapon" , "Aboriginal weapon" , ""It keeps coming back"" , "Self-returning missile" .)