This week's letters thus far (Nos. 59-64, Sun-Wed, Mar 26-29), are an extremely complicated interlace of letters and notes and dramatic narratives within letters; one way of keeping track generally might be to see this little turn as showing us that Clarissa is only driven to Lovelace as an alternative to Solmes. When she is given just a few days peace, she writes immediately that she does not want to see Lovelace; her clear willingness to throw him over ("I am too much alarmed, not to wish and desire, that your letter of this day may conclude all the trouble you have had from, or for, Your humble &c") is partly her triumphing over him and showing him and her who values whom more, but it also show she would be relieved to get rid of him. At the same time we are moving forward to the moment she will leave Harlowe place and the terms of the struggle will change radically.
I don't know if anyone has mentioned that in fact James and Arabella were gambling on Clarissa's running away. They wanted to drive her to this (at least James does) so as to cut her out of the family estate altogether, and perhaps also take back the grandfather's propertyabout which such envy and anger is felt by everyone--how dare the old man give her some leverage against them.