Stallion requiring endless research (4)
I believe the answer is:
stud
'stallion' is the definition.
(stud is a kind of stallion)
'endless research' is the wordplay.
'endless' means to remove the last letter.
'research' becomes 'study'.
'study' with its last letter taken away is 'STUD'.
'requiring' acts as a link.
(Other definitions for stud that I've seen before include "Macho - stable - boss" , "Small piece of jewellery" , "Decoration" , "Horse-breeding establishment; jewellery item" , "Specified arrangement" .)