Lakes are not major highways (6)
I believe the answer is:
broads
'lakes' is the definition.
Although both the answer and definition are plural nouns, I cannot see how they can define each other.
'not major highways' is the wordplay.
'not major' becomes 'b' (I can't justify this - if you can you should believe this answer much more).
'highways' becomes 'roads' (I've seen this in another clue).
'b'+'roads'='BROADS'
'are' is the link.
Can you help me to learn more?
(Other definitions for broads that I've seen before include "found in part of eastern England" , "Old coins" , "Wetland" , "Norfolk holiday area" .)