Speeder's downfall (5)
I believe the answer is:
radar
'speeder's downfall' is the definition.
(I've seen this in another clue)
This is all the clue.
(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" .)