An “intensive” root addition described by Tolkien in The Etymologies from the 1930s (EtyAC/A). It was one of two basic intensive mechanisms, along with the (syllabic) prefixed N- (EtyAC/N). The prefixed vowel a- seems to have been used originally in Primitive Elvish when the base vowel was a, and similarly with E and I (EtyAC/E; Ety/I²); whether this was also true of the vowels o, u is unclear, as Tolkien didn’t mention them. These various vocalic intensifications were frequently accompanied by dynamic lengthening (doubling), with the example given by Tolkien being: ᴹ✶parkā “dry” → ᴹ✶apparkā “very dry, arid” (> N. afarch).
In the case of e- and i-, the examples were dero, dise → ᴹ✶Endero, ᴹ✶Indise “groom, bride”; these examples indicate that other kinds of consonant fortifications were possible, in this case nasalization of stops, which often replaced consonant-doubling for voiced stops in Primitive Elvish.
Specifically in the case of a-, however, it seems it could be used as a general intensive that “was distinct in origin, though similar in function, to the prefixed basic vowel”. Why this was true of a- alone is not clear, but there seems to have been some complex interplay between the vocalic intensives and the intensives derived from syllabic initial ṇ-, with the net result that the intensive prefix in Q. became an-, am-, añ-, depending on the initial consonant.
See the entry on the Quenya comparative for a more detailed discussion of the conceptual development of intensives in Eldarin.
A root in The Etymologies of the 1930s glossed “talk loud or incoherently”, with derivatives like ᴹQ. indyalme “clamour” and N. glamm “barbarous; shouting, confused noise”, the latter serving as the basis for the collective name for Orcs: N. Glamhoth “Barbaric Host” (Ety/GLAM, ÑGAL); this root replace ᴹ√ÑGAL(AM) or ᴹ√ÑYAL(AM) of similar meaning (Ety/ÑGAL; EtyAC/GLAM). In earlier writings, G. Glamhoth was based on glâm “hatred” (GL/39), but in later writings it continued to be translated as “din-horde” or “host of tumult” (UT/39; MR/109; PE17/39), though in the Quendi and Eldar essay S. glam “din, uproar” was derived directly from √(G)LAM “(vocal) sounds that were confused or inarticulate” (WJ/416).
Neo-Eldarin: For purposes of Neo-Eldarin, I think it is worth retaining this root as a (Quenya only?) elaboration of √(G)LAM to salvage ᴹQ. indyalme “clamour”.