Exploding star (4)
'exploding star' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for nova that I've seen before include "Star that suddenly increases in brightness" , "Newly bright star" , "Star that becomes much brighter for short time" , "Star suddenly erupting in brightness" , "It shines" .)
(Other definitions for diva that I've seen before include "Great singer" , "Operatic soprano" , "High-maintenace woman" , "For example Sutherland" , "Prima donna, woman opera singer" .)