Ultimately very loud refusal (6)
I believe the answer is:
rebuff
'refusal' is the definition.
(I have seen 'Blunt refusal' mean 'rebuff' so perhaps 'refusal' could also mean 'rebuff')
'ultimately very loud' is the wordplay.
I cannot quite see how this works, but
'loud' could be 'ff' (a musical term) and 'ff' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for rebuff that I've seen before include "Reject abruptly" , "Refuse" , "Reject bluntly" , "Cold-shoulder" , "Spurn, snub" .)