The headword is a masculine/feminine/neuter comparative form of the adjective ἡδύς , here in the genitive case. See eta 91 for the comparative neuter nominative of the same adjective.
Taken from the Lexicon Ambrosianum (Ambrosianus B12 sup.fol. 77r line 3), which has: συστέλ. ηδιόνος: συγκριτικὸν , “shortens [sc. the penultimate vowel in the genitive] ἡδίονος : comparative”. The origin is perhaps in Choeroboscus, Prolegomena et scholia in Theodosii Alexandrini canones isagogicos de flexione nominum 272, where the adjective is specifically quoted: “The comparatives in ων change the ω to ο in the genitive, as for example… ἡδίων, ἡδιόνος .