Sound barrier (4)
I believe the answer is:
boom
This clue is a double definition.
'sound' is the first definition.
(I know that boom is a more specific form of the action sound)
'barrier' is the second definition.
(I know that boom is a type of barrier)
(Other definitions for boom that I've seen before include "Big bang sound" , "When business is great, opposite to bust" , "Supersonic wave noise" , "The noise of increased business activity?" , "Horizontal spar on board" .)