Travelling by train, for a change. Isn't flying (2,7)
I believe the answer is:
in transit
'travelling' is the definition.
'train for a change isn't flying' is the wordplay.
'for a change' indicates an anagram.
'flying' indicates anagramming the letters (letters fly around into new positions).
'train' with letters rearranged gives 'intra'.
'isnt' anagrammed gives 'nsit'.
'intra'+'nsit'='IN TRANSIT'
'by' is the link.
(Other definitions for in transit that I've seen before include "On its way" , "Being transported" , "during journey" , "(Of goods) being moved" , "on the move?" .)