Horrible old Democrat promises to pay (6)
I believe the answer is:
odious
'horrible' is the definition.
(thesaurus)
'old democrat promises to pay' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'democrat' becomes 'd'.
'promises to pay' becomes 'IOU's' (more than one IOU).
'o'+'d'+'ious'='ODIOUS'
(Other definitions for odious that I've seen before include "Really obnoxious, generally disliked" , "offensive" , "Really hateful, detestable" , "Unpleasant, repulsive" , "Hateful and disgusting" .)