Underwater detection system (5)
I believe the answer is:
sonar
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">underwater detection 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 sonar that I've seen before include "Locating device" , "location device" , "Undersea sensing system" , "System for locating underwater objects" , "Echo-location system" .)