Big bang sound (4)
I believe the answer is:
boom
'big bang sound' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for boom that I've seen before include "Period of prosperity needed" , "Prosperous time -- rumbling sound" , "become suddenly prosperous" , "Prosper vigorously - waterway barrier" , "Bonanza; roar" .)