Bright star (4)
'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 "Exploding star" , "Star briefly brightening" , "Star showing a sudden temporary increase in brightness" , "Faint star that suddenly flares into brightness and then fades again" , "Star that becomes much brighter for short time" .)
'bright star' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sirius that I've seen before include "The Dog Star, brightest of the fixed stars" , "Brightest star in the sky" , "The Dog Star that follows the hunter Orion" , "It's known as the Dog Star" , "The Dogstar" .)