The most common and most likely answer to this crossword clue is the 9 letter word SUPERNOVA
I've seen this crossword clue in The Times
S
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A
More clues leading to the result SUPERNOVA
- Prove sun may explode, leading to a ________?
- Paramount head nervous about a star collapsing
- Star in decline eggs on wicked knight
- Stratford flower seller, America’s rising star
- Prove a sun exploded like this
- Unusually suave porn star
- Terribly nervous drinking quietly with a star
- Marvellous exact opposite of May, perhaps — a bright star
- A sun, over, exploding, having suppressed power
- Star’s late meal half-heartedly eaten in the end
Buddy explains!

The straight part
We know that "________" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ________ = supernova.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does SUPERNOVA mean?
SUPERNOVA noun- The explosion of a star, which increases its brightness to typically a billion times that of our sun, though attenuated by the great distance from our sun. Some leave only debris (Type I); others fade to invisibility as neutron stars (Type II).