Exploding star — a sun? Prove that’s wrong (9)
I believe the answer is:
supernova
'exploding star' is the definition.
(I've seen this before)
'a sun? prove that's wrong' is the wordplay.
'that's wrong' indicates an anagram.
'a'+'sun'+'prove'='asunprove'
'asunprove' anagrammed gives 'SUPERNOVA'.
(Other definitions for supernova that I've seen before include "The sort of star" , "Exploding star that becomes extremely luminous" , "Exploding star that becomes much brighter" , "Star in decline" , "Star that explodes and becomes extremely luminous" .)