Planet discovered in 1781 (6)
I believe the answer is:
uranus
'planet discovered in 1781' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for uranus that I've seen before include "Holst's Magician planet" , "Planet discovered in 1781 by Herschel" , "Ringed planet" , "Planet of solar system" , "Seventh planet from the sun" .)