Disagreeable old duke promises to pay (6)
I believe the answer is:
odious
'disagreeable' is the definition.
(I've seen this before)
'old duke promises to pay' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'duke' becomes 'd' (abbreviation).
'promises to pay' becomes 'IOU's' (promises to pay a debt).
'o'+'d'+'ious'='ODIOUS'
(Other definitions for odious that I've seen before include "Disagreeable" , "Ghastly" , "Unequivocally detestable" , "Thoroughly obnoxious, hateful" , "Hateful, really nasty" .)