Extend thoroughfare through mountain (7)
I believe the answer is:
broaden
'extend' is the definition.
(broadening is a kind of extending)
'thoroughfare through mountain' is the wordplay.
'thoroughfare' becomes 'road' (I've seen this before).
'through' indicates putting letters inside (I've seen this in other clues).
'mountain' becomes 'ben' (ben is a kind of mountain).
'road' placed into 'ben' is 'BROADEN'.
(Other definitions for broaden that I've seen before include "Make wider" , "Stretch" , "Make wide or liberal, perhaps the mind" , "Make more general" , "In one direction, expand" .)