Gang member's beastly sound overwhelming quiet (7)
I believe the answer is:
greaser
'gang member's' is the definition.
(member of a motorcycle gang)
'beastly sound overwhelming quiet' is the wordplay.
'beastly sound' becomes 'grr' (grr is a growling sound).
'overwhelming' is an insertion indicator.
'quiet' becomes 'ease' (I've seen this in another clue).
'grr' placed around 'ease' is 'GREASER'.
(Other definitions for greaser that I've seen before include "An unctuous individual" , "Ship's engineer; motor mechanic" , "Young motorcyclist" , "Mechanic - sycophant" , "Biker" .)