Mark, that tormented saint (5)
I believe the answer is:
stain
'mark' is the definition.
(I know that mark can be written as stain)
'tormented saint' is the wordplay.
'tormented' indicates anagramming the letters.
'saint' anagrammed gives 'STAIN'.
'that' is the link.
(Other definitions for stain that I've seen before include "Discolouring mark" , "Smear" , "Discoloured mark" , "Colour, dye" , "Blot" .)