Sound of Queen once with low backing? (5)
I believe the answer is:
vroom
'sound' is the definition.
(sound made by a car)
'queen once with low backing?' is the wordplay.
'queen once' becomes 'VR' (short for Victoria Regina, Queen Victoria).
'with' says to put letters next to each other.
'low' becomes 'moo' (both can mean to make the noise of a cow).
'backing?' says the letters should be written in reverse (I've seen 'backing' mean this).
'moo' in reverse letter order is 'oom'.
'vr'+'oom'='VROOM'
'of' is the link.
(Other definitions for vroom that I've seen before include "vehicle's noise" , "Fast-moving car noise" , "Sound of a car speeding away" , "loud engine noise" , "roar away" .)