Horse rider's strap (4)
I believe the answer is:
rein
'horse rider's strap' 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 "one's attached to a bit" , "Restraint; strap attached to horse's bit" , "Strap on a bridle" , "Horse's steering strap" , "Horse controller" .)