Not much. In 1958 Morton Smith claimed to have found a portion of a letter written by Clement of Alexandria. It discussed a second edition of the Gospel of Mark, prepared after Peter's death. This second edition supposedly included stories not found in the canonical Mark. The longest of these stories was what appeared to be an alternative account of the resurrection of Lazarus. According to the letter Smith found, this document was kept at Alexandria (of which Mark had been bishop), but not generally disseminated. The Gnostic heretic Carpocrates obtained a copy of the gospel and then revised it, adding his own Gnostic teachings, and then used it to justify the licentious sexual ethics of his followers.
The letter is of dubious authenticity. Smith claimed to have found it handwritten in the back of a book in the library of the Mar Saba monastery in southern Israel. The book itself dated from the 17th century, and the handwriting of the letter was dated from the eighteenth century. Smith published photographs of the letter, but since their publication no other Western scholar has seen the letter.
Even if Smith's account of finding the letter is correct, it is doubtful that the 18th-century person who wrote it in the back of the book had a genuine letter of Clement of Alexandria. He might have composed the letter himself, expecting someone to find it in the future, or he may have had a copy of a letter previously forged in Clement's name.
Even if Clement wrote the letter, it does not prove that the version of Mark he mentions was genuine. Someone between the time of Mark and the time of Clement may have added the additional material and then put forward the Gospel in Mark's name (just as the heretic Carpocrates is supposed to have done). Few scholars who believe Clement wrote the letter believe Mark was the author of the Gospel the letter mentioned. The additional material contains clues that make it unlikely it would have been written by Mark.