Headword and gloss are basically the same word. The former is an older form of the aorist active imperative, second person singular, of proskune/w (see also pi 2747, pi 2748), and the latter a more common equivalent in later Greek.
Adler's note asserts that the entry is generated by the instance of the headword in Sophocles, Philoctetes 776; however, this gloss occurs in a scholion to the instance of it in Aristophanes, Knights 156. (Otherwise unattested.)