Seismic effect (6)
I believe the answer is:
tremor
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">seismic effect</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 tremor that I've seen before include "Nervousness" , "A slight shake, body or earth" , "Vibration - shaking" , "Quaver" , "Small vibration" .)