Loud explosion (4)
(Other definitions for blast that I've seen before include "Strong gust of air" , "slate" , "Severely criticise" , "Effect of explosion" , "Loud noise from a horn" .)
'loud explosion' 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 "hit" , "Completely modern" , "A loud report or a collision" , "Explosive noise" , "Sound of explosion" .)