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