Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
ustor, ōris, m. [uro], a burner of dead bodies, a corpse-burner, Cic. Mil. 33, 90; Cat. 59, 5; Mart. 3, 93, 26; Luc. 8, 738.