Unfortunate stain on fabric (5)
I believe the answer is:
satin
'fabric' is the definition.
(satin is a kind of fabric)
'unfortunate stain' is the wordplay.
'unfortunate' indicates anagramming the letters.
'stain' with letters rearranged gives 'SATIN'.
'on' acts as a link.
(Other definitions for satin that I've seen before include "Smooth glossy fabric" , "Saint (anag.)" , "Stain found in cloth" , "Material with glossy front and dull back" , "Silky material" .)