Control-tower tracking device (5)
I believe the answer is:
radar
'control-tower 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" , "Locator that uses a microwave echo" , "Method for detecting distant planes, ships etc" , "Position detector using radio wave pulses" , "System of detection by echoes" .)