Illusory — ghost (7)
I believe the answer is:
phantom
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">illusory</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >'<span class="explanation-format__original">ghost</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that ghost can be written as phantom)</span><br/ ><br/ ></span>
(Other definitions for phantom that I've seen before include "Illusion (at the opera?)" , "Ballet-dancing costume" , "shadow of his former self?" , "Wraith" , "Something creepy" .)