Firecracker sound (4)
'firecracker sound' 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 "Fringe - report" , "Sound of explosion" , "Loud noise; fringe" , "Vigorous blow" , "A loud report or a collision" .)
(Other definitions for pop that I've seen before include "Cartoon sailor (showing amazement?)" , "Thus goes the weasel in children's rhyme" , "Youthful music for dad?" , "Old jingle - . . . goes the weasel" , "Palindrome of music for the young" .)