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' says to put 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" .)