Brightening star (4)
I believe the answer is:
nova
'brightening 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 that suddenly becomes brighter for a time" , "Star suddenly erupting in brightness" , "Sudden light increase" , "Star that ejects a cloud, thus becoming brighter" , "It shines" .)