Expanse of sea (5)
I believe the answer is:
ocean
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">expanse of 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 "eg Arctic" , "A lot of water!" , "Pacific, for example" , "Vast sea" , "Extensive sea area" .)