One is disturbed by the racket (5)
I believe the answer is:
noise
'the racket' is the definition.
(racket can mean a loud noise)
'one is disturbed' is the wordplay.
'disturbed' indicates an anagram (the order of letters is disturbed).
'one'+'is'='oneis'
'oneis' is an anagram of 'NOISE'.
'by' is the link.
(Other definitions for noise that I've seen before include "Sound, disagreeable perhaps" , "Static - sound" , "Public talk" , "Loud or disturbing sound" , "Din, cacophony" .)