Friends knocked over by explosive sound immediately (4-4)
I believe the answer is:
slap-bang
'immediately' is the definition.
(I've seen this before)
'friends knocked over by explosive sound' is the wordplay.
'friends' becomes 'pals' (pal is a kind of friend).
'knocked over' says the letters should be written in reverse.
'by' is a charade indicator (letters next to each other).
'explosive sound' becomes 'bang' (I've seen this in another clue).
'pals' reversed gives 'slap'.
'slap'+'bang'='SLAP-BANG'
(Other definitions for slap-bang that I've seen before include "Absolutely (in the middle)" , "right?" , "Suddenly - directly" , "All at once - violently" .)