Great sea (5)
I believe the answer is:
ocean
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">great sea</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for ocean that I've seen before include "Huge body of water" , "Large amount of water and" , "Pacific, for example" , "lot of liquid" , "An immense expanse of sea" .)