Supersonic speed measure (4)
I believe the answer is:
mach
'supersonic speed measure' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mach that I've seen before include "Austrian physicist, gave name to speed no." , "Number referring to speed of sound" , "Ratio used to measure high speeds" , "-- Number relates to speeds" , "The speed of sound is ... one" .)