Learning well, horse breeder promises to pay (8)
I believe the answer is:
studious
'learning well' is the definition.
'horse breeder promises to pay' is the wordplay.
'horse breeder' becomes 'stud' (a stud farm breeds horses).
'promises to pay' becomes 'ious' (more than one iou).
'stud'+'ious'='STUDIOUS'
(Other definitions for studious that I've seen before include "Diligent; scholarly" , "Bookish, sedulous" , "showing diligence" , "Like a good schoolboy" , "with great care" .)