Shock a quiet chum (5)
I believe the answer is:
appal
'shock' is the definition.
(I know that shock can be written as appal)
'a quiet chum' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'chum' becomes 'pal' (both can mean a friend).
'a'+'p'+'pal'='APPAL'
(Other definitions for appal that I've seen before include "Fill with horror" , "Greatly horrify" , "Disgust" , "Petrify" , "Greatly dismay" .)