Rebounding sound (4)
I believe the answer is:
echo
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">rebounding sound</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for echo that I've seen before include "A natural sound reflection" , "from Brussels?" , "Send back" , "In The Maid of the Mountains" , "one catches on the rebound?" .)