Harness strap (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">harness strap</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)