Loudness unit (7)
I believe the answer is:
decibel
'loudness unit' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for decibel that I've seen before include "I relate to volume" , "Noise unit" , "Unit of measurement for loudness" , "Unit relating to sound or power measurement" , "Measure of noise intensity" .)