After Beethoven's Ninth, I thunderously hum (4)
I believe the answer is:
niff
'hum' is the definition.
(I've seen this before)
'after beethoven's ninth i thunderously' is the wordplay.
'after' says to put letters next to each other.
'beethoven's ninth' becomes 'n' (9th letter of 'beethoven').
'thunderously' becomes 'ff' (I can't justify this - if you can you should believe this answer much more).
'n' next to 'i' is 'ni'.
'ni'+'ff'='NIFF'
Can you help me to learn more?
(Other definitions for niff that I've seen before include "Hum" , "a stink" .)