This root appears in the Quendi and Eldar essay from 1959-60 with the sense “walk, go, proceed, travel” (WJ/360). Its derivatives were S. Edhel “Elf” and Q. †Eldo which blended with Elda “Elf”, both originally with the sense “one who goes” (✶edelō): those who left lake Cuiviénen to travel to Valinor. It is also the basis of several words meaning “to go”: T. delia- and Q. lelya-.
Many of Quenya derivatives of this root actually derive from an inversion √LED of the original √DEL. According to the Quendi and Eldar essay, this inversion only occurred in Quenya (WJ/363). In earlier writings like The Etymologies, however, ᴹ√LED “go, fare, travel” was the basic root in all languages. It seems that Tolkien revised ᴹ√LED >> √DEL, preserving √LED only as a Quenya variant. The strongest sign of this change was Tolkien’s consistent alteration of N. Eledh >> S. Edhel.
This leaves open the question of what happened to the other non-Quenya derivatives of ᴹ√LED, such as S. edlen(n) “exile”. It is possible that they were transferred to a new root √LEN, which was the basis for S. lembas “way bread” (PE17/60).
A root in etymological notes from 1968, meaning “to will with conscious purpose, immediate or remote”; it was distinct from “be willing, to assent, consent, agree”, which partakes of the will but also involves accident or change (NM/231). Given the many other uses to which √DEL was assigned, odds are this was a transient idea.