Train routes are endlessly modified (8)
I believe the answer is:
eurostar
'train' is the definition.
(I've seen this before)
'routes are endlessly modified' is the wordplay.
'endlessly' means to remove the last letter.
'modified' indicates anagramming the letters.
'routes'+'are'='routesare'
'routesare' with its final letter taken off is 'routesar'.
'routesar' with letters rearranged gives 'EUROSTAR'.
(Other definitions for eurostar that I've seen before include "travel service" , "International train" , "Passenger rail service from London to Paris, etc" , "High-speed rail link between the UK and France" , "International traveller" .)