Central pillar of a spiral staircase (5)
I believe the answer is:
newel
'central pillar of a spiral staircase' is the definition.
(I've seen this before)
This is all the clue.
(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" .)