Sailor on route overseas (6)
I believe the answer is:
abroad
'overseas' is the definition.
('abroad' can be a synonym of 'overseas')
'sailor on route' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'on' says to put letters next to each other.
'route' becomes 'road' ('road' can be a synonym of 'route').
'ab'+'road'='ABROAD'
(Other definitions for abroad that I've seen before include "Pole would be so" , "not in Britain" , "Overseas; (of rumour) widely current" , "In foreign land" , "out of UK?" .)