Scoundrel promises to pay -- that's odd (7)
I believe the answer is:
curious
'odd' is the definition.
(thesaurus)
'scoundrel promises to pay' is the wordplay.
'scoundrel' becomes 'cur' (I've seen this before).
'promises to pay' becomes 'ious' (more than one iou).
'cur'+'ious'='CURIOUS'
'that's' is the link.
(Other definitions for curious that I've seen before include "Intrigued" , "Strange, unusual" , "Odd, rum" , "Eager to learn, strange" , "with a great deal of interest!" .)