Passing nurse in van (9)
I believe the answer is:
transient
'passing' is the definition.
(I know that passing can be written as transient)
'nurse in van' is the wordplay.
'nurse' becomes 'en' (abbreviation for enrolled nurse).
'in' means one lot of letters goes inside another.
'van' becomes 'transit' (Ford Transit van).
'en' put into 'transit' is 'TRANSIENT'.
(Other definitions for transient that I've seen before include "Passing, short-lived" , "Lasting little time" , "Temporary worker" , "Ephemeral" , "on the move" .)