Seismic sea wave (7)
I believe the answer is:
tsunami
'seismic sea wave' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for tsunami that I've seen before include "Large tidal wave" , "Great, fast-moving sea wave" , "Large ocean wave" , "Huge wave caused by earth tremor" , "Huge tidal wave that causes great damage and loss of life" .)