Rider's seat (6)
I believe the answer is:
saddle
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">rider's seat</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 saddle that I've seen before include "load" , "Burden; seat" , "Impose a task on -- assign responsibility for" , "Cut of meat" , "Rider's seat" .)