unexplored land (8)
(Other definitions for terra incognita that I've seen before include "abode of monsters once - with 17d" , "riots in strange land" , "Unknown territory" , "country that's never been surveyed" , "unsurveyed land" .)
(Other definitions for frontier that I've seen before include "Land border of a country" , "wild area" , "Border separating two countries" , "Limit" , "Land-border of a country" .)