Another Splendour on his mouth alit, That mouth, whence it was wont to draw the breath Which gave it strength to pierce the guarded wit, And pass into the panting heart beneath With lightning and with music: the damp death Quench'd its caress upon his icy lips; And, as a dying meteor stains a wreath Of moonlight vapour, which the cold night clips, It flush'd through his pale limbs, and pass'd to its eclipse.
As in the last stanza, we are admiring the body of Adonais-Keats. This time, the speaker focuses on his mouth, which holds "Splendour" (magnificence). The words the youth spoke were apparently quite special.
Those words help defend him against the "guarded wit" of his critics. Keep your eyes peeled for more mentions of these critics, who—as you may remember—were the cause of Keats' death (in Shelley's eyes).
Keats' final words are now dying on his lips. In the same way that the speaker described his dreams as moving around the body while looking for life, he describes Keats' last words with yet another simile: "as a dying meteor."
"Damp death" has arrived, and the words pass through the body to its "eclipse," or ending. Like the sun is eclipsed by the moon, death eclipses the body's life.
The words leave a small trace, though, the same way a meteor leaves a line of light in the sky (like a "wreath," he says). That trace of life makes it look like he's still alive.