Extremely unpleasant party brought over promissory notes (6)
I believe the answer is:
odious
'extremely unpleasant' is the definition.
(I've seen this before)
'party brought over promissory notes' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'brought over' says the letters should be written in reverse.
'promissory notes' becomes 'ious' (IOU is an example).
'do' backwards is 'od'.
'od'+'ious'='ODIOUS'
(Other definitions for odious that I've seen before include "'Repugnant, foul (6)'" , "'Hateful, repulsive (6)'" , "Abominable, detestable" , "Unpleasant, repulsive" , "Really hateful, detestable" .)