One on horseback (5)
I believe the answer is:
rider
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">one on horseback</span>' is the definition.<br/ ><span class="explanation-format__explanation">(someone riding a horse)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for rider that I've seen before include "Equestrian - clause" , "Jury's recommendation" , "extra weight needed for balance?" , "Person on horseback" , "Supplementary clause" .)