Is it permissible to describe a well-known actress and dancer who prides herself on her work (dancing and performing indecent roles) as unchaste or an adulteress, knowing that she claims to be Muslim, and would that fall under the category of slandering chaste women?

Backbiting is originally forbidden, except in specific cases. Among these is someone who openly commits sins; it is permissible to backbite him without dispute among scholars. The proof for this is the Prophet's (peace and blessings be upon him) statement about a man: "Permit him in, what a wretched brother of his tribe (or son of his tribe)." As for accusing someone of adultery, it is not permissible except with clear evidence, which is the presence of four witnesses who explicitly saw the act of adultery.