Symbolism, Imagery, Allegory
Her Pen is Mightier Than [Lovelace's] Sword
From the start, we get that Clarissa likes to write. It's essential that she keeps throwing down on paper, to the extent that she hides her pens and papers from the Harlowe family. So why is Clarissa such a great penpal?
First of all, Clarissa's story is never supposed to get out. Ruined women aren't supposed to talk, according to the Harlowes. But Clarissa practically has a whole volume of letters telling the story of how she did everything right, and it still ended up all wrong. It's more of a tragedy to tell it that way, but at least Clarissa has a voice. Scratch that—she has a pen that tells it like it is. Like Belford writes to Lovelace, "Give sorrow words" (419.16).
Shuttin' It Down
It's not just significant that Clarissa wants to keep writing. It's also pretty telling that Lovelace and the Harlowe family are dead set against her putting pen to paper. What's the big deal, might you ask? Check out what her Uncle Antony has to say: "You had better not write to us, or to any of us. To me, particularly, you had better never to have set pen to paper on the subject whereupon you have written" (32.39).
Say what? Seems like Antony wants to prevent Clarissa from having the ability to communicate her true emotions. Denying her a pen is all about denying her speech and self-determination. If she can't write it, she can't be it.
Fallin' to Pieces
That's why her writing gets so crazy after her rape. When Lovelace rapes her, he takes away her ability to make her own choices; he destroys her self.
Eventually, writing helps her put herself back together. It's important not only that she produce some writing, but that it gets read by someone outside of Mrs. Sinclair's brothel. Otherwise, there's a chance that Clarissa's story will die along with her—because she is her story.
The Pen Keeps Keepin' On
Clarissa sees something in Belford that makes her trust him, which is why she trusts him with distributing eleven letters of forgiveness after her death. It's clear that Clarissa wants to deliver a message from beyond the grave (although she totally could have played the ghost card. That would be awesome).
Need proof why Belford is her perfect representative? While our guy is thinking about Clarissa's terrible fate, he tells Lovelace "I drop my trembling pen" (500.54).
Rather than recording his own version of the narrative, Belford allows Clarissa to tell the whole story in her own words.