Seismic effect (6)
I believe the answer is:
tremor
'seismic effect' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tremor that I've seen before include "Nervousness" , "A slight shake, body or earth" , "Vibration - shaking" , "Quaver" , "Small vibration" .)