A secondary highway overseas (6)

I believe the answer is:

abroad

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'overseas' is the definition.
(synonyms)

'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).
'ab'+'road'='ABROAD'

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" .)

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