Very loud, but ultimately quiet (4)
I believe the answer is:
soft
'quiet' is the definition.
(similar in meaning)
'very loud but ultimately' is the wordplay.
'very' becomes 'so' (both can mean 'extremely').
'loud' becomes 'f' (short for forte - 'loud' in music).
'ultimately' says to take the final letters.
The final letter of 'but' is 't'.
'so'+'f'+'t'='SOFT'
(Other definitions for soft that I've seen before include "Native of Riyadh" , "Not hard, rough or harsh" , "Malleable, light" , "Easily yielding to pressure" , "Yielding, easily cut" .)