Train isn't diverted on the way (2,7)
I believe the answer is:
in transit
'on the way' is the definition.
(I've seen this in another clue)
'train isn't diverted' is the wordplay.
'diverted' is an anagram indicator (letters must be diverted to different positions).
'train'+'isnt'='trainisnt'
'trainisnt' with letters rearranged gives 'IN TRANSIT'.
(Other definitions for in transit that I've seen before include "Being transported" , "while travelling" , "on the wagon?" , "On its way" , "on the move?" .)