messing with your theme again :-)
The strings play with their mutes in this quiet slow movement, thus giving the clarinet the chance to sing over soft strings.
In his youth, Brahms spent a few years barnstorming Europe with the hotdogging violinist Eduard Remenyi, who often played the violin upside down, and as a result he had his fill of gypsy violin music. When Brahms wrote gypsy music in an instrumental context, he wrote it for piano (Piano Quartet in G minor, Op. 25) or viola (String Quintet in G, Op. 111). The central panel here is a long haunting gypsy passage for clarinet backed by muted strings.