Pretty wide for a secondary highway (5)
I believe the answer is:
broad
'pretty wide' is the definition.
'broad' can be an answer for 'wide' (synonyms). I'm unsure of the 'pretty' bit.
'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'
'for a' is the link.
Can you help me to learn more?
(Other definitions for broad that I've seen before include "Covering a wide range" , "Wide; general" , "River expansion" , "Spacious - tolerant" , "East Anglian waterway" .)