___ Hammarskjöld, only posthumous winner of a Nobel Peace Prize

DAG

This Swedish diplomat and UN Secretary-General shares his name with a Scandinavian word for "day."

This clue refers to Dag Hammarskjöld, the Swedish diplomat who served as the second Secretary-General of the United Nations and was awarded the Nobel Peace Prize in 1961 after his death. The blank in the clue signals that the answer is his first name, "DAG," which fits the three-letter space. The mention of "posthumous winner" is a direct hint to his unique distinction in Nobel Prize history.