This short poem from Clark Ashton Smith (CAS) references the passing of one of the giants of Greek mythology, famous for his role in the war between the Giants and the Olympian gods.
Earlier in his career, CAS wrote several poems about the Titanomachy, or the war between the Titans and the Olympians. The Gigantomachy in which Enceladus was involved was a separate event, but another milestone in the Olympians' quest to make their power absolute.
All of that said, it appears that in "Passing of An Elder God" CAS is following John Keats' lead, for in the abandoned long poem "Hyperion", Keats has Enceladus as one of the Titans, rather than as one of the Giants:
Upon his elbow rais’d, all prostrate else,
Shadow’d Enceladus; once tame and mild
As grazing ox unworried in the meads;
Now tiger-passion’d, lion-thoughted, wroth,
He meditated, plotted, and even now
Was hurling mountains in that second war,
Not long delay’d, that scar’d the younger Gods
To hide themselves in forms of beast and bird.
The defeat of this mighty figure at the hands of the Olympians informs CAS' brief poem, which seems to have echoes as well of Poe's "The Bells", using end rhymes with a hint of onomatopoeia ("clangorous", "monotonous") suggesting the reverberations of a tolling bell.