Tarnished (6)
(Other definitions for impure that I've seen before include "Contaminated or licentious" , "Sullied or adulterated" , "Assign a value to" , "Unclean" , "Dirty or immoral" .)
(Another definition for dulled that I've seen is " Dimmed".)
'tarnished' is the definition.
(synonyms)
This is all the clue.