Speeder's downfall (5)
I believe the answer is:
radar
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">speeder's downfall</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 radar that I've seen before include "A help in locating" , "Means of detecting and locating distant objects" , "Instrument to detect and locate distant objects" , "System for detecting objects with radio waves" , "Pulse detection system" .)