A secondary highway overseas (6)
I believe the answer is:
'overseas' is the definition.
'a secondary highway' is the wordplay.
'a secondary' becomes 'ab' (I am not sure about this - if you are sure you should believe this answer much more).
'highway' becomes 'road' (highway is a type of road).
Can you help me to learn more?
(Other definitions for abroad that I've seen before include "in France, say" , "out of UK?" , "In another country, not at home" , "At large - overseas" , "At large; widely current" .)