How to get to Brussels? Possibly, routes are endless (8)
I believe the answer is:
eurostar
'how to get to brussels?' is the definition.
'possibly routes are endless' is the wordplay.
'possibly' indicates anagramming the letters.
'endless' means to remove the last letter.
'are' with its final letter removed is 'ar'.
'routes' anagrammed gives 'eurost'.
'eurost'+'ar'='EUROSTAR'
(Other definitions for eurostar that I've seen before include "Chunnel service" , "Waterloo to Paris train" , "High-speed rail link between the UK and France" , "Channel link" , "Railway service via the Channel Tunnel" .)