Atlantic, e.g. (5)
I believe the answer is:
ocean
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">atlantic eg</span>' is the definition.<br/ ><span class="explanation-format__explanation">(atlantic is a kind of ocean)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for ocean that I've seen before include "eg Indian, Atlantic" , "A mighty sea" , "Main" , "Pacific, perhaps" , "Immense expanse or quantity" .)