Killer whales (5)
I believe the answer is:
orcas
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">killer whales</span>' is the definition.<br/ ><span class="explanation-format__explanation">('orca' can be a synonym of 'killer whale')</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for orcas that I've seen before include "Killers" , "Killer whales" .)