A hormone, something orbiting larger body (8)
I believe the answer is:
asteroid
'something orbiting larger body' is the definition.
(rocky body orbiting the sun)
'a hormone' is the wordplay.
'hormone' becomes 'steroid' (steroid hormone is a kind of hormone).
'a'+'steroid'='ASTEROID'
(Other definitions for asteroid that I've seen before include "Small celestial body moving around the sun" , "space traveller" , "Any of the small planets revolving around the sun" , "Minor planet" , "Bit of rock that moves around the heavens" .)