English Antonyms Practice Test 8

Hidden : kept out of sight, concealed.
Surreptitious : kept secret, especially because it would not be approved of.
Obscure : not discovered or known about, uncertain.
Exposed : make (something) visible by uncovering it.
Hidden, Surreptitious, Obscure and Underhand are more or less synonyms to each other.

So, antonym of Underhand is Exposed.