The most common and most likely answer to this crossword clue is the 8 letter word PINWHEEL
I've seen this crossword clue in The Times
P
I
N
W
H
E
E
L
Buddy explains!

The straight part
We know that "firework" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case firework = pinwheel.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does PINWHEEL mean?
PINWHEEL noun- An artificial flower with a stem, usually plastic, for children: the flower spins round in the wind, like a small paper windmill.
- A firework which forms a kind of spinning wheel.
- A cogged (toothed) gear.
- A pastry which resembles the artificial flowers above, with some filling or topping in the center.
- Any food product consisting of layers (for example of pastry and sweet filling, or of bread and meat) rolled into a spiral, visually similar to a cinnamon roll.
- To spin.