Awful notes after party upset (6)
I believe the answer is:
odious
'awful' is the definition.
(thesaurus)
'notes after party upset' is the wordplay.
'notes' becomes 'ious' (IOU is an example**).
'after' says to put letters next to each other.
'party' becomes 'do' (do is a kind of party).
'upset' says the letters should be written in reverse.
'do' reversed gives 'od'.
'ious' put after 'od' is 'ODIOUS'.
(Other definitions for odious that I've seen before include "Loathsome and repugnant" , "Detestable, greatly disliked" , "Replusive, abhorrent (6)" , "Extremely unpleasant" , "Contemptible" .)