Firework expert extremely eager (7)
I believe the answer is:
cracker
'firework' is the definition.
(cracker is a kind of firework)
'expert extremely eager' is the wordplay.
'expert' becomes 'crack' (as in 'a crack team').
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'eager' with its centre removed is 'er'.
'crack'+'er'='CRACKER'
(Other definitions for cracker that I've seen before include "Thin crisp biscuit" , "Beautiful woman" , "it may be specially for Christmas" , "pulled at Christmas?" , "example for solver?" .)