Major motorway intersects back roads (7)
I believe the answer is:
seminal
'major' is the definition.
(I've seen this before)
'motorway intersects back roads' is the wordplay.
I cannot quite see how this works, but
'motorway' could be 'mi' (resembles M1, English motorway) and 'mi' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for seminal that I've seen before include "Strongly influential" , "Highy important" , "Original and influential" , "Groundbreaking" , "Major" .)