Insult a loud appearance (7)
I believe the answer is:
affront
'insult' is the definition.
(synonyms)
'a loud appearance' is the wordplay.
'loud' becomes 'f' (short for forte - 'loud' in music).
'appearance' becomes 'front' (front is a kind of appearance).
'a'+'f'+'front'='AFFRONT'
(Other definitions for affront that I've seen before include "Rant off over insult" , "Insult, offend" , "A slight" , "not trivial offence" , "provocation" .)