One is appalled by the racket (5)
I believe the answer is:
noise
'the racket' is the definition.
(racket can mean a loud noise)
'one is appalled' is the wordplay.
'one is' can be anagrammed to 'NOISE'.
However, I am not sure how the anagram is indicated.
'by' is the link.
Can you help me to learn more?
(Other definitions for noise that I've seen before include "Row" , "Loud or disturbing sound" , "Virgil's birthplace" , "Clatter" , "Fuss" .)