There was a set of possible verb forms lelya, nelya, delta, delya (the last two probably primitive forms) appearing in a note probably from the mid-1960s, with a difficult-to-read gloss that seems to be something like “thick, thicken & congeal” according to Christopher Gilson (PE17/17). This note included the root √DEL “thick, dense”; the form nelya hints at a possible root change to √NDEL, or it could some sort of assimilation/disimilation of ancient d to n rather than the usual change of initial d to l. All these verb forms were rejected along with the root √DEL.
Neo-Quenya: Since we have no other words of similar meaning in Tolkien’s later writings, I would salvage ᴺQ. nelya- “to thicken, congeal” for purposes of Neo-Quenya, and assume it is derived from a Neo-Root ᴺ√NDEL “thick, dense”.
nelya adj. "third" (VT42:25; also neldëa). Pl. Nelyar "Thirds", the original name of the Teleri (or rather the direct Quenya descendant of the original Primitive Quendian name) (WJ:380).