Passing trains, ten diverted (9)
I believe the answer is:
transient
'passing' is the definition.
('transient' can be a synonym of 'passing')
'trains ten diverted' is the wordplay.
'diverted' indicates an anagram (each letter get diverted elsewhere).
'trains'+'ten'='trainsten'
'trainsten' anagrammed gives 'TRANSIENT'.
(Other definitions for transient that I've seen before include "fugitive" , "Passing, short-lived" , "gone in a trice!" , "Fleeting" , "Ephemeral" .)