Heavenly body doesn't need a body-building chemical (7)
I believe the answer is:
steroid
'body-building chemical' is the definition.
(type of chemical that may be used by bodybuilders)
'heavenly body doesn't need a' is the wordplay.
'heavenly body' becomes 'asteroid' (asteroid is a kind of heavenly body).
'doesn't need' suggests deleting specific letters.
'asteroid' with 'a' removed is 'STEROID'.
(Other definitions for steroid that I've seen before include "Organic compound" , "Compond such as many hormones and vitamins" , "growth medication" , "Hormone perhaps" , "muscle-building compound" .)