Horse? Hard-working? No promises (4)
I believe the answer is:
stud
'horse?' is the definition.
(I know that stud is a type of entire)
'hard-working? no promises' is the wordplay.
'hard-working?' becomes 'studious' (I've seen this before).
'no' indicates named letters should be taken away.
'promises' becomes 'IOUs' (promises to pay a debt).
'studious' with 'ious' removed is 'STUD'.
(Other definitions for stud that I've seen before include "Part of a football boot" , "pin" , "Randy young male" , "kind of farm" , "Projecting boss" .)