Crash barrier (4)
I believe the answer is:
boom
This clue is a double definition.
'crash' is the first definition.
(I've seen this before)
'barrier' is the second definition.
(thesaurus)
(Other definitions for boom that I've seen before include "Period of prosperity needed" , "it sometimes accompanies 19 [BABY]" , "Spar - loud noise" , "Prosperous time - sound of explosion - yachting spar" , "The noise of increased business activity?" .)