Doppler device (5)
I believe the answer is:
radar
'doppler device' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for radar that I've seen before include "Range-finding apparatus" , "Palindromic tracking system" , "Range and direction finding equipment" , "Measuring device that locates distant objects" , "System for detecting objects with radio waves" .)