The headword as transmitted appears to be third person singular, present indicative, of a verb, fli/w, onomatopoetic with sounds made by clawed birds (i.e. birds of prey); but see next note.
[1] Diffferent glossing, according to Adler, in the Ambrosian Lexicon (390), but there is a broadly similar entry ('an sound imitative of clawed [birds]') in other lexica; references at Photius phi224, where Theodoridis obelizes the transmitted headword and cites Dindorf and Cobet for the opinion that it is a corruption of fnei/ (see LSJ s.v.). The Photius entry ends by citing 'Aristophanes'; this is fr. 885 Kock, now 914 K.-A.