It’s an unfortunate mark (5)
I believe the answer is:
stain
'mark' is the definition.
('stain' can be a synonym of 'mark')
'it's an unfortunate' is the wordplay.
'unfortunate' is an anagram indicator.
'its'+'an'='itsan'
'itsan' anagrammed gives 'STAIN'.
(Other definitions for stain that I've seen before include "Discoloration" , "Smear" , "Unsightly mark" , "Discolouration" , "Penetrative dye; mark" .)