Ebb and flow of the sea (4)
I believe the answer is:
tide
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">ebb and flow of the sea</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for tide that I've seen before include "Something going in and out" , "Current, ebb" , "Periodic rise and fall of the sea" , "What might carry" , "Rising and falling of the sea" .)