Space station on time -- that's an illusion (6)
I believe the answer is:
mirage
'that's an illusion' is the definition.
(I've seen this in another clue)
'space station on time' is the wordplay.
'space station' becomes 'Mir' (I've seen this in another clue).
'on' means one lot of letters go next to another.
'time' becomes 'age' (an age is a period of time).
'mir'+'age'='MIRAGE'
(Other definitions for mirage that I've seen before include "fantasy" , "Optical illusion, in desert perhaps" , "Hallucination caused by hot air" , "It doesn't exist" , "unreal" .)