Cite the scanned version of the original dicionary like this:
Toller, T. Northcote, and Joseph Bosworth. "fæsten-geat." An Anglo-Saxon Dictionary: Based on the Manuscript Collections of the Late Joseph Bosworth : Supplement. Oxford: Clarendon Press, 1921. 202.
, es; n. A fortress or city gate; arcis vel urbis porta :-- Wið ðæs fæstengeates folc onette the people hastened to the city gate, Judth. 11; Thw. 23, 38; Jud. 162.