An opaque entry, and made the more so because it appears in other lexica in different forms: in Photius the lemma itself is the adjective only, i.e. δίκην is lacking; the Synagoge has δίκην as the first part of the gloss. All that seems cerain, therefore, is that ἀγοραίαν (accusative case) is quoted from somewhere.
The glossing term dikaiologia can mean either a defense plea or a forensic speech of any kind: see LSJ s.v.