Secondary highway, but wide (5)
I believe the answer is:
broad
'wide' is the definition.
(synonyms)
'secondary highway' is the wordplay.
'secondary' becomes 'b' (I can't justify this - if you can you should believe this answer much more).
'highway' becomes 'road' (highway is a type of road).
'b'+'road'='BROAD'
'but' is the link.
Can you help me to learn more?
(Other definitions for broad that I've seen before include "Covering a wide range" , "River expansion" , "East Anglian waterway" , "not detailed" , "dame" .)