Sonar signal
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sonar signal</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for echo that I've seen before include "Mythical nymph" , "Reflection of sound waves" , "Reflection of sound wave" , "repetitive" , "Audio effect" .)