Who did Agamemnon leave in charge of Clytemnestra and what did Aegisthus do with this person?

In Book 3, Agamemnon left the family minstrel in charge of his wife, Clytemnestra.

Nestor tells Telemachus that Aegisthus had had his eye on Clytemnestra ever since Agamemnon went away to war. To win over Clytemnestra, Aegisthus constantly plied her with words of love. Eventually Clytemnestra succumbed to his attentions, and the two became lovers.

Meanwhile, Aegisthus lured the minstrel (or bard) to a desert island and abandoned him to birds of prey. With the minstrel dead and out of the way, Aegisthus had complete sway over Clytemnestra. In due time, Aegisthus killed Agamemnon as well. In revenge, Orestes (the son of Agamemnon) killed Aegisthus and his mother, Clytemnestra. Nestor holds up Orestes as a good example for Telemachus to follow, and he advises him not to stay away from his home for too long, especially when his property and all that he holds dear are in jeopardy.