Equestrian's strap (4)
I believe the answer is:
rein
'equestrian's strap' 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 "Restriction" , "Strap on a bridle" , "Hold back" , "Part of tack" , "Long strap to restrain" .)