Such a body, one may imagine, is out of this world! (6)
I believe the answer is:
astral
'such' is the definition.
Although both the answer and definition are adjectives, I can't see how they can define each other.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for astral that I've seen before include "Like the stars" , "Starry sort of altars." , "Relating to stars" , "Related to the stars" , "Universal" .)