Not a major route to CO (6)
I believe the answer is:
abroad
'not a major route to co' is the definition.
I can't judge whether this defines the answer.
'not a major route to co' is the wordplay.
'not' is a deletion indicator.
'major route' becomes 'a road' (I've seen this before).
'to' says to put letters next to each other (I've seen this in other clues).
'co' becomes 'ab' (I am not sure about this - if you are sure you should believe this answer much more).
'aroad' with 'a' taken out is 'road'.
'road' after 'ab' is 'ABROAD'.
(Other definitions for abroad that I've seen before include "over a wide area" , "At large - overseas" , "In foreign land" , "In another country, not at home" , "In a foreign country" .)