Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Subertāni, ōrum, m., a people of Etruria, the inhabitants of Subertum, the modern Sovretto in Tuscany, Plin. 3, 5, 8, § 52.
Hence, Subertānus, a, um, adj., of or belonging to the Subertani: forum, Liv. 26, 23, 5.