Edits one nasty comment in the margin (8)
I believe the answer is:
sidenote
'comment in the margin' is the definition.
(note at the side of a page)
'edits one nasty' is the wordplay.
'nasty' is an anagram indicator (a nasty or wrong spelling).
'edits'+'one'='editsone'
'editsone' anagrammed gives 'SIDENOTE'.
(Other definitions for sidenote that I've seen before include "Marginal comment" , "I could be pencilled in for the wing" .)