Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
postŭmo, āre, v. n. [postumus], to come after, be inferior (eccl. Lat.): alicui, Tert. Apol. 19: omnis consummatio, etsi ordine postumat, effectu tamen anticipat, id. Res. Carn. 45.