The headword is a very rare neuter noun, attested only in lexicography (etc.) and always, as here, in the nominative/accusative singular. (LSJ note it as a variant of pro/exma, though in fact that is even rarer.) Same glossing in other lexica (references at Photius pi1207 Theodoridis); and Eustathius (in his Commentary on Homer's Odyssey) explicitly mentions the Suda entry.