Deadly, fatal (6)
I believe the answer is:
mortal
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">deadly</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that deadly can be written as mortal)</span><br/ ><br/ >'<span class="explanation-format__original">fatal</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ ></span>
(Other definitions for mortal that I've seen before include "Human being, subject to death" , "Very drunk < person" , "Fatal" , "Subject to death, like all mankind" , "Certain to die" .)