Central pillar of a spiral staircase (5)
I believe the answer is:
newel
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">central pillar of a spiral staircase</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 newel that I've seen before include "Top/bottom supporting post of a stair-rail" , "Support post for stair rail" , "(Of a flight) upright post" , "Column" , "Post at base of staircase" .)