Of stars in fast rally cars (6)
I believe the answer is:
astral
'of stars' is the definition.
(relating to stars or astronomy)
'in fast rally cars' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ASTRAL' can be found hidden inside 'fast rally cars'.
(Other definitions for astral that I've seen before include "Related to the stars" , "Starry sort of altars." , "Resembling the stars - of the spirit world" , "Relating to stars" , "Like the stars" .)