Vile party backs fiscal promises (6)
I believe the answer is:
odious
'vile' is the definition.
(I've seen this in another clue)
'party backs fiscal promises' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'backs' says the letters should be written backwards.
'fiscal promises' becomes 'ious' (promises to pay a debt 'I owe you').
'do' reversed gives 'od'.
'od'+'ious'='ODIOUS'
(Other definitions for odious that I've seen before include "Extremely unpleasant" , "Really hateful, abhorrent" , "Repellent" , "Unpleasant, repulsive" , "Hateful, generally despised" .)