Stuff that makes a nasty stain on a piece of fur (7)
I believe the answer is:
fustian
'stuff' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'makes a nasty stain on a piece of fur' is the wordplay.
'makes' becomes 'u' (I can't justify this - if you can you should believe this answer much more).
'a nasty' indicates anagramming the letters (a nasty or wrong spelling).
'on' means one lot of letters go next to another.
'a piece of' suggests taking the first letters.
The initial letter of 'fur' is 'f'.
'stain' with letters rearranged gives 'stian'.
'u'+'stian'='ustian'
'ustian' put after 'f' is 'FUSTIAN'.
'that' is the link.
Can you help me to learn more?
(Other definitions for fustian that I've seen before include "Bombast" , "Bombastic (style)" , "Thick twilled cotton cloth, or pompous language" , "Hard-wearing twilled cloth -- it's faun (anag)" , "Hard-wearing fabric; pompous language" .)