Illusory — ghost (7)
I believe the answer is:
phantom
I believe this is a double definition.
'illusory' is the first definition.
(I've seen this in another clue)
'ghost' is the second definition.
(I know that ghost can be written as phantom)
(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" .)