Measure in decibel levels (3)
I believe the answer is:
ell
'measure' is the definition.
(historical unit of length)
'in decibel levels' is the wordplay.
'in' indicates a hidden word.
'ELL' is hidden in the letters of 'decibel levels'.
(Other definitions for ell that I've seen before include "distance" , "Once, forty-five inches" , "Old measure equivalent to about 45 inches in England" , "Embellish with old measure of length" , "Old 45-inch measure" .)