Planet demoted in 2006 (5)
I believe the answer is:
pluto
'planet demoted in 2006' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for pluto that I've seen before include "Solar system body" , "recently losing status within star system" , "Greek god - cartoon dog" , "Ninth planet of solar system" , "it's another world there" .)