Instrument to track distant objects (5)
I believe the answer is:
radar
'instrument to track distant objects' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for radar that I've seen before include "Range-finding apparatus" , "one sends out signals" , "It can locate things in the dark or fog" , "Range and direction finding equipment" , "Aircraft detection system" .)