Extremely negative V&A star (9)
I believe the answer is:
supernova
'star' is the definition.
(I know that supernova is a type of star)
'extremely negative va' is the wordplay.
'extremely' becomes 'super' (synonyms).
'negative' becomes 'no' (no is a kind of negative).
'super'+'no'+'va'='SUPERNOVA'
(Other definitions for supernova that I've seen before include "Star made brilliant by stellar explosion" , "Star in decline" , "Exploding star that becomes extremely luminous" , "The sort of star" , "cosmic explosion" .)