A quiet friend finds fish horrific (9)
I believe the answer is:
appalling
'horrific' is the definition.
(I've seen this before)
'a quiet friend finds fish' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'friend' becomes 'pal' (pal can mean a friend).
'finds' is a charade indicator (letters next to each other).
'fish' becomes 'ling' (ling is a kind of fish).
'a'+'p'+'pal'+'ling'='APPALLING'
(Other definitions for appalling that I've seen before include "page entirely covered in copying" , "Terrible" , "Awful" , "Shockingly bad" , "Absolutely horrifying" .)