Chief route to Tube station (7)
I believe the answer is:
archway
'tube station' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'chief route' is the wordplay.
'chief' becomes 'arch' (synonyms).
'route' becomes 'way' (way is a kind of route).
'arch'+'way'='ARCHWAY'
'to' is the link.
Can you help me to learn more?
(Other definitions for archway that I've seen before include "Vaulted entrance; London tube station" , "Curved entrance" , "Vaulted passage" , "entrance, perhaps" , "part of London" .)