Sentence about revolutionary R&B sound (6)
I believe the answer is:
timbre
'sound' is the definition.
(a particular musical sound or tone)
'sentence about revolutionary rb' is the wordplay.
'sentence' becomes 'time' (a prison sentence).
'about' is an insertion indicator.
'revolutionary' indicates an anagram.
'rb' with letters rearranged gives 'br'.
'time' placed around 'br' is 'TIMBRE'.
(Other definitions for timbre that I've seen before include "Quality of musical sound" , "Musical tone quality" , "Tone-colour" , "Tone of voice - sounds wooden" , "Ring" .)