The unglossed headword is masculine accusative singular or neuter nominative/accusative singular of the rare adjective ka/tauxmos (not in LSJ; cf. Sophocles s.v.). Outside the Suda, its extant occurrences are in fact confined to Theophylact Simocatta (twice in the Histories, once in the Letters), so this headword is very probably quoted from Letter 8. (If so, it is neuter accusative, used as a substantive.)
[1] An approximation of Theophylact Simocatta, Histories 5.4.1, on the Median city of Singara (using the dative singular of the headword); cf. de Boor (194) and Whitby (136). Theophylact's text in fact reads toi=s polemi/ois a)na/lwton: "unassailable to their enemies". The preceding text explains that Chosroes II (cf. chi 418 note) had sequestered his wives and children at Singara (spring of 591) on the assumption that the lack of available water in the region would greatly hinder any assault on the city. The lexicographer's emendation is not without motivation, however, since an ancient city with a tenuous water supply would indeed be quite vulnerable to a prolonged siege. On Singara (present-day Sinjar, Iraq) see Barrington Atlas map 89 grid D4.