Sympathetic vibration (9)
I believe the answer is:
resonance
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sympathetic vibration</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 resonance that I've seen before include "Sound amplification" , "Deep, clear, ringing quality" , "shaking" , "Plangency" , "Sonority" .)