Tracking system (5)
I believe the answer is:
radar
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">tracking system</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for radar that I've seen before include "Position detector using radio wave pulses" , "System for detecting objects with radio waves" , "one sends out signals" , "Method for detecting distant planes, ships etc" , "Technique using beam" .)