The sound of a piano is excruciating (5)
I believe the answer is:
noise
'the sound' is the definition.
(a noise is a sound)
'of a piano is excruciating' is the wordplay.
'of' indicates a hidden word.
'NOISE' is hidden within 'piano is excruciating'.
(Other definitions for noise that I've seen before include "Rumour" , "Loud sound; bruit" , "Clamour" , "Sound of any kind" , "Row" .)