A noun in the Gnomish Lexicon of the 1910s given as G. agla “flash” (GL/42); Tolkien made a point that it was not connected to G. augla “ray of sunlight, sunbeam” (GL/20), and thus it was likely derived from the early root ᴱ√KALA (QL/44). In the Gnomish Lexicon Slips it was revised from {aglan or agl >>} aglen “a flash” (PE13/108). In Early Noldorin Word-lists of the 1920s this became {aglen >>} ᴱN. aglann “ray of light” (PE13/136, 158).

Neo-Sindarin: I think this word can be adapted to Neo-Sindarin as ᴺS. agol “flash”, where the -ol is the usual result of a syllabified final l: *agl(a) > agḷ > agol. I would adapt the pre-revision Gnomish form because I think the Early Noldorin form is better as ᴺS. aglan “ray of light”, and agol also happens to compatible with early 1950s ✶ak’la- “shine out, flash” (PE18/87).