The brightest star in the sky (6)
I believe the answer is:
sirius
'the brightest star in the sky' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sirius that I've seen before include "It's known as the Dog Star" , "The Dog Star, brightest of the fixed stars" , "The Dog Star that follows the hunter Orion" , "Brightest star in the sky" , "The Dogstar" .)