Same entry (but with the apparently more authentic single-l version of the name) in Harpocration and Photius.
Derkylidas [OCD(4) 442] was not, as these lexica claim, a Spartan king, but rather a Spartiate who won distinction in several military commands during the late fifth and early fourth centuries. Xenophon reports that he won a reputation for cleverness and deception which earned him the nickname "Sisyphos" (Xenophon, Hellenica 3.1.8). Derkylidas first appears in 411, when he led an expedition that seized the strategic city of Abydos on the Hellespont from the Athenians (Thucydides 8.61-62). He then apparently stayed at Abydos as harmost, or governor, but was disgraced in some argument with the Persian satrap Pharnabazos in 407/6 (Xen. Hell. 3.1.9). From 399 to 397 he commanded the Spartan army (composed of freed helots, allied Peloponnesians, and the remnants of Xenophon's ten thousand) which was engaged in hostilities against the Persian satraps Tissaphernes and Pharnabazos in Ionia and Hellespontine Phrygia (Xen. Hell. 3.1.8-2.20). Replaced in command by the Spartan king Agesilaos II in 396, Derkylidas apparently returned to Sparta and fought in the battle of the Nemea in 394 (Xen. Hell. 4.3.2). He brought the news of this Spartan victory to Agesilaos, who sent him back to the Hellespont. Derkylidas arrived at Abydos just before the Spartan naval disaster at Knidos, after which he was responsible for maintaining Spartan control of the Hellespont (Xen. Hell. 4.8.3-5). He was relieved of the governorship of Abydos in 389 (Xen. Hell. 4.8.32).
Despite his considerable military reputation, he was apparently looked down on to some degree because of his status as a bachelor, in a Spartan state which encouraged marriage (Plutarch Lycurgus 15.2).