Plane-tracking device (5)
I believe the answer is:
radar
'plane-tracking device' 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 "Palindromic tracking system" , "System for detecting things using radio waves" , "Ranging system" , "Instrument to detect and track distant objects" , "Locator that uses a microwave echo" .)