Step one made inside heavenly body (5)
I believe the answer is:
stair
'step' is the definition.
(a single step in a staircase)
'one made inside heavenly body' is the wordplay.
'one made' becomes 'i' (I can't explain this - if you can you should believe this answer much more).
'inside' is an insertion indicator.
'heavenly body' becomes 'star' (star is a kind of heavenly body).
'i' going inside 'star' is 'STAIR'.
Can you help me to learn more?
(Other definitions for stair that I've seen before include "One step of a flight" , "One step in a series" , "Way between floors" , "one's seen in flight" , "Flight unit" .)