Get on (a horse) (5)
I believe the answer is:
mount
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">get on a horse</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for mount that I've seen before include "Horse; climb" , "Climb - Everest, for example" , "Climb - eg Everest" , "Take < a back" , "hill" .)