Awful-sounding clan abuse (8)
I believe the answer is:
diatribe
'abuse' is the definition.
(I've seen this before)
'awful-sounding clan' is the wordplay.
'awful' becomes 'dire' (synonyms).
'sounding' indicates a 'sounds like' (homophone) clue.
'clan' becomes 'tribe' (synonyms).
'dire' is a homophone of 'dia'.
'dia'+'tribe'='DIATRIBE'
(Other definitions for diatribe that I've seen before include "Long speech" , "stream of abuse" , "Bitter critical attack" , "'Abusive, bitter speech (8)'" , "Abusive harangue" .)