Jet speed measure (4)
I believe the answer is:
mach
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">jet speed measure</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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." .)