Sly devil loses one pound then promises to pay (7)
I believe the answer is:
devious
'sly' is the definition.
(thesaurus)
'devil loses one pound then promises to pay' is the wordplay.
'loses' is a deletion indicator.
'one pound' becomes 'il'.
'then' is a charade indicator (letters next to each other).
'promises to pay' becomes 'ious' (more than one iou).
'devil' with 'il' taken out is 'dev'.
'dev'+'ious'='DEVIOUS'
(Other definitions for devious that I've seen before include "Underhand, deceitful" , "Tricky or shifty" , "Underhand -- sneaky" , "Machiavellian, roundabout" , "Going astray" .)