A second-class thoroughfare overseas (6)
I believe the answer is:
abroad
'overseas' is the definition.
('abroad' can be a synonym of 'overseas')
'a second-class thoroughfare' is the wordplay.
'second class' becomes 'b'.
'thoroughfare' becomes 'road' (I've seen this before).
'a'+'b'+'road'='ABROAD'
(Other definitions for abroad that I've seen before include "Stare lecherously at (someone)" , "Away from home" , "In foreign land" , "Europe, for example" , "in Paris, perhaps" .)