Sea movement (4)
I believe the answer is:
tide
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sea movement</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 "turning point" , "Powerful surge" , "Moon-driven sea movement" , "Surge of events" , "Season; current" .)