Physicist Ernst (4)
I believe the answer is:
mach
'physicist ernst' 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 a body's speed to the speed of sound" , "-- Number relates to speeds" , "Ratio used to measure high speeds" , "Ratio of the speed of a moving body to the speed of sound" , "Number referring to speed of sound" .)