Incredible; illusory (6)
I believe the answer is:
unreal
I believe this clue is a double definition.
'incredible' is the first definition.
(thesaurus)
'illusory' is the second definition.
(similar in meaning)
(Other definitions for unreal that I've seen before include "Existing only in the imagination, not actual" , "Illusory, imaginary" , "Illusory; amazing" , "Phantasmagorical" , "Not factual" .)