About Saint Mark? (5)
I believe the answer is:
stain
'mark?' is the definition.
(I know that mark can be written as stain)
'about saint' is the wordplay.
'about' is an anagram indicator.
'saint' anagrammed gives 'STAIN'.
(Other definitions for stain that I've seen before include "Discoloration" , "Discolour - blemish" , "Satin (anag.)" , "Dirty mark; dye" , "Unsightly mark" .)