Equestrian strap (4)
I believe the answer is:
rein
'equestrian 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 "given free to experienced riders?" , "Director's power" , "Free ... is carte blanche" , "Part of a horses's harness" , "Hold back" .)