Diamond firework (8)
I believe the answer is:
sparkler
This clue is a double definition.
'diamond' is the first definition.
(something which sparkles)
'firework' is the second definition.
(sparkler is a kind of firework)
(Other definitions for sparkler that I've seen before include "Diamond (colloq.)" , "Possibly brilliant" , "Firework or diamond ring" , "Jewel" , "Firework - precious stone" .)