Deathly (6)
I believe the answer is:
deadly
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">deathly</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that deadly can be written as deadly)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for deadly that I've seen before include "sniper aspires to be" , "Lethal or mortal" , "Lethal, could be fatal" , "Lethal, capable of causing fatality" , "Lethal, capable of causing fatal injury" .)