Very bright star (4)
I believe the answer is:
nova
'very bright star' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nova that I've seen before include "Star briefly brightening" , "Star that suddenly increases in brightness" , "Exploding star" , "Star that becomes much brighter for short time" , "Star showing a sudden, brief increase in luminosity" .)