But now, thy youngest, dearest one, has perish'd, The nursling of thy widowhood, who grew, Like a pale flower by some sad maiden cherish'd, And fed with true-love tears, instead of dew; Most musical of mourners, weep anew! Thy extreme hope, the loveliest and the last, The bloom, whose petals nipp'd before they blew Died on the promise of the fruit, is waste; The broken lily lies—the storm is overpast.
The speaker is still addressing Urania. Now he spends some time examining her grief and her early life with Adonais.
In the version of the myth Shelley uses for this poem (there are plenty of versions of Adonais' story), Urania was made into a widow after Adonais' birth. He was pretty much all she had. No wonder she's so distraught.
Her tears nourished her child like a flower, he says. The speaker is using another simile here.
He's also calling on nature to mourn their "loveliest" creation, who died before it could bear "fruit." What kind of fruit can a man bear? Well, he could be referring to more of Keats' writing, or he could mean children.
Either way, he calls this premature death a "waste."
The broken lily is a metaphor for something cut down at its prime. The young man was at his most beautiful, like the lily. The storm that killed the lily has past, but we are still left with its dead body.
Notice a few tongue-twisters in these lines ("Most musical of mourners")? The "Sound Check" section has more examples of Shelley's sonic tricks and wordplay.