A nasty smear can spoil the paper (5)
I believe the answer is:
reams
'spoil the paper' is the definition.
Although both the answer and 'spoil' are nouns, they are not in the same form of the noun. However, I know that 'paper' relates to this answer.
'a nasty smear' is the wordplay.
'a nasty' is an anagram indicator (a nasty or wrong spelling).
'smear' with letters rearranged gives 'REAMS'.
'can' acts as a link.
Can you help me to learn more?
(Other definitions for reams that I've seen before include "Large quantity of written matter" , "Lots of paper" , "Large quantities (of paper?)" , "This much paperwork" , "Scrapes the barrel" .)