Sudden loud noise (4)
I believe the answer is:
bang
'sudden loud noise' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for bang that I've seen before include "Sudden noise; fringe" , "Sudden loud noise like an explosion" , "A loud report or a collision" , "Report - something the hairdresser did?" , "Sudden loud noise (often 'big')" .)