With sun in orbit one means to ascend (5)
I believe the answer is:
stair
'with sun in orbit one' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'means to ascend' is the wordplay.
'means' becomes 'air ' (I have seen 'Means of travelling' mean 'air ' so perhaps 'means' could also mean 'air ').
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'ascend' becomes 'st' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'air' after 'st' is 'STAIR'.
Can you help me to learn more?
(Other definitions for stair that I've seen before include "One step of a flight" , "Tarsi (anag)" , "Bit of a flight!" , "One of a 15" , "this could get one down" .)