A sound step (8)
I believe the answer is:
footfall
I believe this clue is a double definition.
'a sound' is the first definition.
(footfall is a kind of sound)
'step' is the second definition.
(I know that step can be written as footfall)
(Other definitions for footfall that I've seen before include "Commercial patronage" , "Sound of someone walking" , "Sound of step" , "Customers" .)