Cite the scanned version of the original dicionary like this:
Toller, T. Northcote, and Joseph Bosworth. "ealdor-apostol." An Anglo-Saxon Dictionary: Based on the Manuscript Collections of the Late Joseph Bosworth : Supplement. Oxford: Clarendon Press, 1921. 762.
, aldor-apostol, es; m. The chief apostle, the chief of the apostles; princeps apostōlōrum :-- He mynster getimbrede on áre Sce Petres ðæs ealdorapostoles he built a monastery in honour of St. Peter, the chief apostle, Bd. 4, 18; S. 586, 26.