Shock a quiet friend (5)
I believe the answer is:
appal
'shock' is the definition.
(I know that shock is a kind of appal)
'a quiet friend' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'friend' becomes 'pal' (pal can mean a friend).
'a'+'p'+'pal'='APPAL'
(Other definitions for appal that I've seen before include "Fill with horror" , "To shock, dismay" , "expressed dismay" , "Put at rest (suspicion)" , "Petrify" .)