Factory worker achieving speed of sound in back street (9)
I believe the answer is:
machinist
'factory worker' is the definition.
(I've seen this before)
'speed of sound in back street' is the wordplay.
'speed of sound' becomes 'mach' (I've seen this before).
'back street' becomes 'ist' (I can't justify this - if you can you should believe this answer much more).
'mach'+'in'+'ist'='MACHINIST'
'achieving' is the link.
Can you help me to learn more?
(Other definitions for machinist that I've seen before include "factory worker, perhaps" , "Eg, lathe operator" , "Engineer" , "tool operator" , "One working on a device" .)