Wide waters but not major highways (6)
I believe the answer is:
broads
'not major highways' is the definition.
Although both the answer and definition are plural nouns, I cannot see how one could define the other.
'wide waters' is the wordplay.
'wide' becomes 'broad' ('broad' can be a synonym of 'wide').
'waters' becomes 's' (I can't justify this - if you can you should believe this answer much more).
'broad'+'s'='BROADS'
'but' acts as a link.
Can you help me to learn more?
(Other definitions for broads that I've seen before include "Wetland" , "Old coins" , "found in part of eastern England" , "Norfolk holiday area" .)