Briefly scrutinise horse for breeding (4)
I believe the answer is:
stud
'horse for breeding' is the definition.
(male horse for breeding)
'briefly scrutinise' is the wordplay.
'briefly' means to remove the last letter.
'scrutinise' becomes 'study' (I've seen this in another clue).
'study' with its final letter removed is 'STUD'.
(Other definitions for stud that I've seen before include "Variety of poker" , "Football boot peg" , "This farm breeds horses" , "kind of farm" , "seen in a boiled shirt" .)