"Underline a relative clause.” This challenge would give a lot of adults the sweats. It would even—whisper it—flummox many professional writers and editors.
https://www.economist.com/books-and-arts/2019/03/30/the-agony-and-ecstasy-of-grammar

Underline a relative clause. って、関係詞節に下線を引け、という意味ですか？
もしそうなら、なにが難しいのだろう？