A quiet friend went ahead, horrified (8)
I believe the answer is:
appalled
'horrified' is the definition.
('appal' can be a synonym of 'horrify')
'a quiet friend went ahead' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'friend' becomes 'pal' (pal can mean a friend).
'went ahead' becomes 'led' (led the way).
'a'+'p'+'pal'+'led'='APPALLED'
(Other definitions for appalled that I've seen before include "Aghast, horrified" , "Greatly dismayed" , "Dismayed - aghast" , "Scandalised" , "Shocked and horrified" .)