Hardworking boss promises to pay (8)
I believe the answer is:
studious
'hardworking' is the definition.
(someone studious works hard at their studies)
'boss promises to pay' is the wordplay.
'boss' becomes 'stud' (both can mean a metal decoration).
'promises to pay' becomes 'IOU's' (more than one IOU).
'stud'+'ious'='STUDIOUS'
(Other definitions for studious that I've seen before include "Devoted to learning; deliberate" , "Thoughtful - hard-working" , "showing diligence" , "reading a lot" , "Like a good schoolboy" .)