Surge (6,4)
There are a few possibilities for this:
rising tide
'surge' is the definition.
(I have seen 'Sudden surge' mean 'inrush' so perhaps 'surge' could also mean 'inrush')
This is all the clue.
(Other definitions for inrush that I've seen before include "Internal thrust" , "Sudden arrival (of people or water?)" , "Rapid flow from outside" , "Overwhelming flow" , "Sudden arrival or entry" .)
'surge' is the definition.
(I know that onrush is a type of surge)
This is the entire clue.
(Other definitions for onrush that I've seen before include "Swarm" , "Forward surge" , "Attack" , "Career" , "Forward dash" .)
'surge' is the definition.
(I know that billow is a kind of surge)
This is all the clue.
(Other definitions for billow that I've seen before include "Wave; upward flow" , "Swell out in the wind" , "Become inflated; rise up, as in waves" , "Swell out, as a flag" , "Swell out (as a sail)" .)
(Another definition for rising tide that I've seen is " between low and high water".)