A drug to get a heavenly body (8)
I believe the answer is:
asteroid
'heavenly body' is the definition.
(asteroid is a kind of heavenly body)
'a drug to get a' is the wordplay.
'a drug' becomes 'steroid' (type of drug).
'to get' is a charade indicator (letters next to each other).
'steroid' after 'a' is 'ASTEROID'.
(Other definitions for asteroid that I've seen before include "Any of the small planets revolving around the sun" , "Ceres, say" , "Minor planet" , "Small celestial body composed of rock and metal" , "Small planet orbiting the Sun" .)