Tail (6)
I believe the answer is:
shadow
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">tail</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that tail can be written as shadow)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for shadow that I've seen before include "Trail" , "Dark area from which light is blocked" , "It's cast when light is blocked" , "Partial or complete darkness" , "Unilluminated area" .)