The word is most probably a corrupted reading, and although AND1’s definition of ‘assurance’ (sub estance² of the printed version) was tentatively suggested by the editor of Lib Alb (p.248 n.4), no etymological evidence supports this. The same passage in Liber Horn reads ‘fraunce’, which the editor of Lib Alb interprets as ‘clearance by frankpledge’ (again lacking etymological evidence).

One possibility is to read the word as a corruption of fiaunce (see fiance, and the locution prendre fiance de: ‘to receive allegiance from’).