Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
sustentātus, ūs, m. [sustento], a holding up or upright, a sustaining, support (postclass.): levia sustentatui, gravia demersui, App. Mag. p. 287, 34: cujus ille sustentatu, Aus. Per. 5.