Supersonic unit (4)
I believe the answer is:
mach
'supersonic unit' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mach that I've seen before include "-- Number relates to speeds" , "Austrian physicist, gave name to speed no." , "Number referring to speed of sound" , "Ratio of the speed of a moving body to the speed of sound" , "Ratio of a body's speed to the speed of sound" .)