Breeding establishments keep promise to be hard working (8)
I believe the answer is:
studious
'hard working' is the definition.
Both the answer and definition are adjectives. Maybe you can see a link between them that I can't see?
'breeding establishments keep promise' is the wordplay.
'breeding establishments' becomes 'studs' (I am not sure about this - if you are sure you should believe this answer much more).
'keep' means one lot of letters goes inside another.
'promise' becomes 'iou' (I owe you, a promise to pay a debt).
'studs' placed around 'iou' is 'STUDIOUS'.
'to be' is the link.
Can you help me to learn more?
(Other definitions for studious that I've seen before include "possessing industry" , "Bookish, scholarly" , "with great care" , "Given to serious reading" , "Deliberate and painstaking - preparing for exam?" .)