Imaginary; amazing (6)
I believe the answer is:
unreal
I believe this clue is a double definition.
'imaginary' is the first definition.
(similar in meaning)
'amazing' is the second definition.
(unreal can informally mean amazing or incredible)
(Other definitions for unreal that I've seen before include "Not actual or credible" , "Fanciful" , "Artificial, insubstantial" , "Lacking in substance" , "Illusory, imaginary" .)