Lead-free substance akin to vapour trail, leaving one sort of plane (6)
I believe the answer is:
astral
'sort of plane' is the definition.
(astral plane is a mystical concept)
'lead-free substance akin to vapour trail leaving one' is the wordplay.
I cannot quite see how this works, but
'substance' could be 'ra' (Ra is an example) and 'ra' is found within the answer.
'one' could be 'a' and 'a' is found within the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for astral that I've seen before include "like a star" , "Related to the stars" , "To do with stars on altars" , "Starry sort of altars." , "Relating to stars" .)