It comes straight from the horse's mouth (4)
I believe the answer is:
rein
'it comes straight from the horse's mouth' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rein that I've seen before include "leading article?" , "Check; control" , "Restraint; strap attached to horse's bit" , "Strap on a bridle" , "Controlling strap" .)