Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
insertĭo, ōnis, f. [2. insero], a putting in, ingrafting, grafting (post-class.), Isid. Orig. 17, 6, 2: oleastri, Aug. Ep. 120, 20: surculorum, Macr. S. 1, 7, 25.