Whale or dolphin (8)
I believe the answer is:
cetacean
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">whale</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">dolphin</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for cetacean that I've seen before include "Encase cat with mammals like whales and dolphins" , "Generic name for whale, dolphin or porpoise" , "Animal order including whales and dolphins" , "Member of the animal order containing whales and dolphins" , "One swimming" .)