Firework plant (6)
I believe the answer is:
rocket
I believe this clue is a double definition.
'firework' is the first definition.
(I know that rocket is a firework)
'plant' is the second definition.
(I know that rocket is a type of herb)
(Other definitions for rocket that I've seen before include "Salad vegetable" , "Firework and lettuce" , "Self-propelled firework or missile" , "Go up very quickly" , "Salad plant; missile" .)