Fizz fizzing endlessly (4)
I believe the answer is:
asti
'fizz' is the definition.
'fizzing endlessly' is the wordplay.
'fizzing' becomes 'astir' (both can mean excited).
'endlessly' means to remove the last letter.
'astir' with its last letter removed is 'ASTI'.
(Other definitions for asti that I've seen before include "Sparkling wine named after an Italian Piedmont town" , "Alcohol" , "Sparkling drink" , "Italian 'sparkler'" , "Italian town, famous for its wine" .)