Traveller to admit following motorway (7)
I believe the answer is:
migrant
'traveller' is the definition.
(migrant is a kind of traveller)
'admit following motorway' is the wordplay.
'admit' becomes 'grant'.
'following' is a charade indicator (letters next to each other).
'motorway' becomes 'MI' (resembles M1, English motorway).
'grant' after 'mi' is 'MIGRANT'.
'to' is the link.
(Other definitions for migrant that I've seen before include "Animal moving seasonally" , "Moving across territories" , "Person moving countries" , "Animal moving with the seasons" , "Itinerant" .)