Towards the end of the novel, Clarissa is in the process of hosting her party when she learns from her guests, the Bradshaws, of a man having jumped from a window earlier that day Prior to this Clarissa had been feeling off about her party when it's described "she did think it mattered, her party, and it made her feel quite sick to know that it was all going wrong, all falling flat."(164) After hearing of Dr. Bradshaw's patient, Clarissa goes into an empty room and thinks "what business had the Bradshaws to talk of death at her party?" (179)
The next few pages show that Clarissa is really affected by the death of this stranger, who we know to be Septimus. "Always her body went through it first, when she was told, suddenly, of an accident;" (179)-- "They went on living (she would have to go back; the rooms were still crowded; people kept on coming). They (all day she had been thinking of Bourton, of Peter, of Sally), they would grow old."--Clarissa articulates the otherwise unspoken fact that this young man will stay young forever but life goes on as usual for others. (180) Clarissa and the other people around her will continue to get older but Septimus won't. In the next few sentences she then establishes that "death is defiance" and also an "attempt to communicate." She wonders if Septimus "plunged holding his treasure", which I take to mean if he killed himself while still feeling content with life.
I don't articulate well what they mean but these are a lot of thoughts coming from just a brief mention of suicide. I think Clarissa's focus on Septimus' death stems from her love of life, the reason why she values her parties. "Defiance" of living is so different from how Clarissa approaches the world around her that she tries to imagine what Septimus' thought process (communication, treasure) was that led him to his end. She then has many thoughts about her own life with the topic of death in mind.
Septimus' death being an impetus for Clarissa to examine her mortality was discussed in class, and the language such as that on page 182 fit with this idea well. "He made her feel the beauty; made her feel the fun." (182) The paragraph this line is from suggests to me that Septimus' interruption of Clarissa's focus on personal and relationship-related events makes her realize that life altogether is precious and that she should appreciate what she gets to experience, even if she has regrets and doubts. Her earlier rumination on her party being a failure isn't ever returned to, and the novel ends with Clarissa returning to the party and seeing Peter Walsh. Presumably they can now talk to each other more in depth.
