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