A fastener for the breeding establishment (4)
I believe the answer is:
stud
I believe this clue is a double definition.
'a fastener' is the first definition.
(I've seen this before)
'breeding establishment' is the second definition.
(I've seen this before)
'for the' acts as a link.
(Other definitions for stud that I've seen before include "Projecting boss" , "Stallion for breeding - fastener" , "Large-headed nail" , "gambling with cards" , "Randy young male" .)