Large expanse of sea (5)
I believe the answer is:
ocean
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">large expanse of 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 ocean that I've seen before include "Large expanse of water" , "Major sea" , "A mighty sea" , "Great sea" , "Huge sea" .)