Two sailors with an unknown part of Asia (7)
I believe the answer is:
tartary
'part of asia' is the definition.
(I know that Tartary is part of Asia)
'two sailors with an unknown' is the wordplay.
'two sailors' means two substitutions for 'sailor'.
'sailor' becomes 'tar' (slang term for a sailor).
'sailor' becomes 'tar' (slang term for a sailor).
'with' means one lot of letters go next to another.
'an unknown' becomes 'y' (algebra).
'tar'+'tar'+'y'='TARTARY'
(Other definitions for tartary that I've seen before include "central Eurasian area" , "historical region long ago" .)