Powerful ocean surge (5,4)
I believe the answer is:
tidal wave
'powerful ocean surge' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tidal wave that I've seen before include "natural disaster?" , "Very powerful movement (at sea?)" , "Overwhelming surge" , "Tsunami" , "Huge movement of public opinion" .)