The headword is a masculine noun in the nominative (and vocative) singular; see generally LSJ s.v. pre/sbis (A). It is presumably generated from its occurrence in the genitive singular at Aristophanes, Acharnians 93 (see next note): Dicaeopolis expresses the sarcastic wish that a crow might peck out the eye of Pseudo-Artabas and of the ambassador as well.
The genitive's proparoxytone (on the antepenult) accent, noted by the Suda, is an exception; cf. Smyth 163 and another instance of this declension at kappa 1021.
[1] Following the scholia to Aristophanes, Acharnians 93 (web address 1), which contend that pre/sbews is the genitive case for pre/sbis, not for presbeu/s; cf. Grammatici Graeci, pp. 232-3.
[2] The material that now follows = Photius, Lexicon pi1152 Theodoridis, on the genitive plural forms of the masculine nouns presbeu/s and presbeuth/s (ambassador, envoy); see generally LSJ s.vv. It is unclear why oratory is specified: both these forms, esp. the first, are commonplace elsewhere too.
[3] This common form is the accusative plural of the aforementioned masculine noun presbeuth/s.