Lustful Sarah, with Bill, promises to pay (9)
I believe the answer is:
salacious
'lustful' is the definition.
(thesaurus)
'sarah with bill promises to pay' is the wordplay.
'sarah' becomes 'sal' (Sally is a common pet form of Sarah).
'with' means one lot of letters go next to another.
'bill' becomes 'ac'.
'promises to pay' becomes 'ious' (more than one iou).
'sal'+'ac'+'ious'='SALACIOUS'
(Other definitions for salacious that I've seen before include "Lewd" , "Arousing lustful feelings" , "Steamy" , "Arousing lustful thoughts" , "Prurient, bawdy" .)