St. Clement's is a church in London, England near Gracechurch Street. This is the church where Lydia claims that she and Wickham got married: "We were married you know, at St. Clement's, because Wickham's lodgings were in that parish. And it was settled that we should all be there by eleven o'clock. My uncle and aunt and I were to go together" (Austen 282).

Austen, Jane. Pride and Prejudice, edited by Robert P. Irvine, 2nd ed., Broadview Press, 2020. 

The following is a photograph of St. Clement's. This is the same building depicted in drawings that are from closer to Austen's lifetime.

A more contemporaneous picture to Austen's time of St. Clements Danes Church in London.