Discolour (5)
(Other definitions for stain that I've seen before include "Spot - shame" , "Discoloured mark" , "Discolour - blemish" , "Discolouring mark" , "Black mark for saint" .)
'discolour' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tarnish that I've seen before include "Taint" , "(Of metal) lose lustre" , "Spoil" , "Film" , "Stain or blemish, perhaps a reputation" .)