Star is circling in empty space (5)
I believe the answer is:
sinus
'empty space' is the definition.
'sinus' can be an answer for 'space' (I have seen 'Hollow space' mean 'sinus' so perhaps 'space' could also mean 'sinus'). I am not sure about the 'empty' bit.
'star is circling in' is the wordplay.
'star' becomes 'sus' (I can't explain this - if you can you should believe this answer much more).
'is circling' indicates putting letters inside (circling can mean going around or containing).
'sus' going around 'in' is 'SINUS'.
Can you help me to learn more?
(Other definitions for sinus that I've seen before include "Cavity in the facial bones" , "Cavity in skull connecting with nostrils" , "Cavity in body (connecting to nose)" , "Air-filled cavity in one's head" , "Hole in the head" .)