Answer:
<em>"loss with which heaven has been pleased to afflict you"</em>
Explanation:
The full sentence in the letter is
<em>"Consider the good lady who has taken you under her roof. . . as your mother, as the only person to whom, since the loss with which heaven has been pleased to afflict you, you can now look up."</em>
Thomas Jefferson, had entrusted the care of his three children (including Patsy) to Thomas Hopkinson, the mother of a friend, before he left for his official duties. He wrote a letter on November 28th, 1783 to eleven-year-old Patsy who was his oldest daughter.
In literature this technique of using mild or indirect reference to some harsh or unpleasant things/event is known as euphemism.