The headword, a one-word adverb in the Greek, is registered by LSJ s.v. with the primary meaning gently, slowly (in Xenophon and elsewhere; however, LSJ's sense II, at close quarters, is the one required here and is illustrated by both of the quotations given. Besides LSJ, see Trapp, p. 1729, s.v. sxe/dhn, in der Nähe (in the vicinity).
[1] The gloss is also an adverb; cf. alpha 4528 (gloss), epsilon 585 (gloss), pi 2617 (end), and see generally LSJ s.v.
Other lexica supplying similar entries are Hesychius (sigma2968, s.v. sxedo/n) and Etymologicum Magnum 739.45 (Kallierges) (s.v. sxedo/n, near, hard by). This same alternative form appears at Homer, Iliad 3.15 (web address 1), where Alexandros/Paris challenges the best of the Argives to individual combat as soon as the Trojan and Achaean forces had drawn near; cf. the corresponding D scholion, which glosses sxedo/n as plhsi/on, e)ggu/s, close, nearby.
[2] Quotation (transmitted, in Adler's view, via the Excerpta Constantini Porphyrogeniti) unidentifiable. On the ambiguity of to/ceuma see LSJ s.v.
[3] Quotation (again, in Adler's view, transmitted via the Excerpta) likewise unidentifiable. In her critical apparatus Adler reports that mss FV read ga\r kai\, For while fighting at close quarters…; also that Bernhardy suggested replacing to\ with ti (anyone) or toi, (then, surely, verily).