The headword is a feminine noun in the nominative singular; see generally LSJ s.v. Usually, though, it appears in the plural fliai/ (cf. Orion [Author, Myth], Etymologicum 160.32), and indeed the present entry is probably generated by Homer, Odyssey 17.221, which has the dative plural flih=si (web address 1); see the scholia there.
[1] Similarly glossed in Hesychius s.v. flia=s (genitive singular) and Etymologicum Magnum 796.19; cf. omicron 826 gloss. Adler also cites Lexicon Ambrosianum 369.