Unfortunate stain on material (5)
I believe the answer is:
satin
'material' is the definition.
(satin is a kind of material)
'unfortunate stain' is the wordplay.
'unfortunate' is an anagram indicator.
'stain' anagrammed gives 'SATIN'.
'on' is the link.
(Other definitions for satin that I've seen before include "Silk fabric" , "covered with?" , "Stain on shiny fabric" , "Shiny solk" , "Rich cloth" .)