A sound measure (7)
I believe the answer is:
decibel
'a sound measure' is the definition.
(I've seen this in another clue)
'measure' is the wordplay.
I cannot really understand how this works, but
'measure' could be 'dec' (Dec is an example) and 'dec' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for decibel that I've seen before include "I relate to volume" , "Noise unit" , "Unit for measuring intensity of sound" , "Measure of sound" , "Unit of measurement for loudness" .)