Endlessly scrutinise the horses (4)
I believe the answer is:
stud
'the horses' is the definition.
(I've seen this before)
'endlessly scrutinise' is the wordplay.
'endlessly' means to remove the last letter.
'scrutinise' becomes 'study' (I've seen this before).
'study' with its last letter taken away is 'STUD'.
(Other definitions for stud that I've seen before include "Football boot's peg-like projection" , "who believes he can keep it up" , "Sex god" , "Group of horses for breeding" , "Large headed nail" .)