Organ interrupted by start of indescribable racket (5)
I believe the answer is:
noise
'racket' is the definition.
(racket can mean a loud noise)
'organ interrupted by start of indescribable' is the wordplay.
'organ' becomes 'nose' (nose is a kind of organ).
'interrupted by' means one lot of letters goes inside another.
'start of' says to take the initial letters.
The initial letter of 'indescribable' is 'i'.
'nose' placed around 'i' is 'NOISE'.
(Other definitions for noise that I've seen before include "interference" , "Fuss" , "Loud or disturbing sound" , "Clatter" , "Rumour" .)