Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
con-vallo, āvi, ātum, 1, v. a.; lit., to surround with an intrenchment; hence, in gen., to surround, encircle (late Lat.): terras omnes Oceani ambitu, Gell. 12, 13, 20.
Trop., Tert. Carm. de Judicio Dom. 6.