Passenger transport crossing space in passing (9)
I believe the answer is:
transient
'passing' is the definition.
(synonyms)
'passenger transport crossing space' is the wordplay.
'passenger transport' becomes 'transit' (transit can mean the transport of people).
'crossing' means one lot of letters goes inside another.
'space' becomes 'en'.
'transit' going around 'en' is 'TRANSIENT'.
'in' acts as a link.
(Other definitions for transient that I've seen before include "gone in a trice!" , "Lasting a short time" , "fugitive" , "Here today, gone tomorrow" , "on the move" .)