Harness strap (4)
I believe the answer is:
'harness 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?" , "Hold back" , "(Horse's) guiding strap" , "Horse controller" , "Leather used for control in horse-riding" .)