Clegg was a Grand Maester of the Citadel who served on the small council under an unknown Targaryen King.
At the end of the First Dornish War, Princess Deria Martell presented to King Aegon I Targaryen a letter from her father. Although the content of the letter was never revealed, many have speculated on its content.
Grand Maester Clegg, who came to King’s Landing many years later, concluded that Dorne no longer had the strength to fight. Clegg suggested Prince Nymor might have threatened that, should his peace be refused, he would engage the Faceless Men of Braavos to kill King Aegon’s son and heir, Aenys, then but six years old. Clegg's suggestion was never proven.