Seas (6)
I believe the answer is:
oceans
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">seas</span>' is the definition.<br/ ><span class="explanation-format__explanation">('ocean' can be a synonym of 'sea')</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for oceans that I've seen before include "Arctic and Indian" , "Vast bodies of water" , "Large amounts (of water)" , "Great seas" , "Large bodies of water" .)