Equestrian's control
I believe the answer is:
rein
'equestrian's control' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rein that I've seen before include "one's attached to a bit" , "Restraint; strap attached to horse's bit" , "Controlling strap" , "Part of tack" , "Director's power" .)