Tree with tip-top heavenly body (6)
I believe the answer is:
planet
'heavenly body' is the definition.
(planet is a type of heavenly body)
'tree with tip-top' is the wordplay.
'tree' becomes 'plane tree' (plane is a type of tree).
'with' is a charade indicator (letters next to each other).
'tip-top' becomes 't' (top of the word 'tip-').
'plane'+'t'='PLANET'
(Other definitions for planet that I've seen before include "body in orbit" , "One orbiting star" , "Venus, for example" , "Sun's orbiter" , "Eg, a ruby" .)