Contemptible cheat goes back on promises to pay (6)
I believe the answer is:
odious
'contemptible' is the definition.
(thesaurus)
'cheat goes back on promises to pay' is the wordplay.
'cheat' becomes 'do'.
'goes back' is a reversal indicator.
'on' says to put letters next to each other.
'promises to pay' becomes 'ious' (more than one iou).
'do' backwards is 'od'.
'od'+'ious'='ODIOUS'
(Other definitions for odious that I've seen before include "Vile" , "Unpleasant, repulsive" , "Hateful, generally despised" , "Comparisons are said to be thus" , "'Repugnant, foul (6)'" .)