Grow dull (7)
(Other definitions for tarnish that I've seen before include "Take the shine off" , "Become dull or blemished" , "Stain like silver" , "Cause to lose lustre, silver or reputation" , "Besmirch" .)
(Other definitions for dim that I've seen before include "Not bright, cloudy" , "Not shining brightly" , "out of mind?" , "thick" , "progressing towards piano" .)