Breeder is hard-working? No promises (4)
I believe the answer is:
stud
'breeder' is the definition.
(I've seen this in another clue)
'hard-working? no promises' is the wordplay.
'hard working?' becomes 'studious' (studious can mean hard-working).
'no' suggests deleting specific letters.
'promises' becomes 'IOUs' (promises to pay a debt).
'studious' with 'ious' taken away is 'STUD'.
'is' is the link.
(Other definitions for stud that I've seen before include "Collar fastener - form of poker" , "Wall timber" , "Collar fastener or horse farm" , "associated with Press" , "Place of breeding" .)