Fed up in the morning, Anto is upset about one slur (10)
I believe the answer is:
defamation
'slur' is the definition.
(slur can mean a defamation or accusation)
'fed up in the morning anto is upset about one' is the wordplay.
'fed up' becomes 'def' ('fed' going up in a down clue).
'in the morning' becomes 'am'.
'is upset' indicates anagramming the letters (I've seen 'are upset' mean this).
'about' means one lot of letters goes inside another.
'one' becomes 'I' (Roman numeral).
'anto' with letters rearranged gives 'aton'.
'aton' placed around 'i' is 'ation'.
'def'+'am'+'ation'='DEFAMATION'
(Another definition for defamation that I've seen is " Assassination of character".)