Painstaking boss promises to pay (8)
I believe the answer is:
studious
'painstaking' is the definition.
(I've seen this in another clue)
'boss promises to pay' is the wordplay.
'boss' becomes 'stud' (both can mean a metal decoration).
'promises to pay' becomes 'IOU's' (promises to pay a debt).
'stud'+'ious'='STUDIOUS'
(Other definitions for studious that I've seen before include "Bookish, scholarly" , "Diligent, assiduous" , "Deliberate and painstaking - preparing for exam?" , "reading a lot" , "Devoted to learning; deliberate" .)