Jet speed measure (4)
I believe the answer is:
mach
'jet 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 "Ratio of the speed of a moving body to the speed of sound" , "Ratio used to measure high speeds" , "Number referring to speed of sound" , "Ratio of a body's speed to the speed of sound" , "Austrian physicist, gave name to speed no." .)