Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
undĕcĭmus, a, um, num. adj. [unusdecimus], the eleventh: legio, Liv. 30, 18, 10: annus, Verg. E. 8, 39: dies, Plin. 11, 54, 118, § 283.