Suddenly bright star (4)
I believe the answer is:
'suddenly bright star' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for nova that I've seen before include "Star showing a sudden, brief increase in luminosity" , "Star suddenly erupting in brightness" , "Star that ejects a cloud, thus becoming brighter" , "Suddenly expanding star" , "sun" .)