The Clue When many a party starts (Hint: Latest eclipse)
The Answer SET
The answer "SET" fits the clue because many parties begin in the evening when the sun sets. The hint "latest eclipse" also relates to the idea of the sun setting, as eclipses happen when the moon blocks the sun's light. So, both the timing of a party and the occurrence of an eclipse connect to the word "set." This makes "SET" a clever answer to the clue.