Peculiar rogue promises to pay (7)
I believe the answer is:
curious
'peculiar' is the definition.
(synonyms)
'rogue promises to pay' is the wordplay.
'rogue' becomes 'cur' (I've seen this in other clues).
'promises to pay' becomes 'IOU's' (more than one IOU).
'cur'+'ious'='CURIOUS'
(Other definitions for curious that I've seen before include "Eager to know something, perhaps nosy" , "Eager to learn, strange" , "Odd or eager to know" , "Intrigued" , "Eager to learn something" .)