Diligent boss promises to pay (8)
I believe the answer is:
studious
'diligent' is the definition.
(similar in meaning)
'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, sedulous" , "Diligent; scholarly" , "Devoted to learning; deliberate" , "showing diligence" , "Deliberate and painstaking - preparing for exam?" .)