Reversal of law concerning prolonged sound (6)
I believe the answer is:
repeal
'reversal of law' is the definition.
'repeal' can be an answer for 'reversal' (I have seen 'reversal of act' mean 'repeal' so perhaps 'reversal' could also mean 'repeal'). I am not sure about the 'of law' bit.
'concerning prolonged sound' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'prolonged sound' becomes 'peal' (peal is a kind of sound. I am not sure about the 'prolonged' bit.).
're'+'peal'='REPEAL'
Can you help me to learn more?
(Other definitions for repeal that I've seen before include "Rescind or withdraw a law" , "reverse" , "set aside" , "'Annul, revoke (6)'" , "Annulment" .)