Unfortunate stain on cloth (5)
I believe the answer is:
satin
'cloth' is the definition.
(satin is a kind of cloth)
'unfortunate stain' is the wordplay.
'unfortunate' indicates anagramming the letters.
'stain' is an anagram of 'SATIN'.
'on' is the link.
(Other definitions for satin that I've seen before include "Rich cloth" , "Stain on silky cloth" , "Fine material" , "Glossy textile" , "covered with?" .)