Four-sided tapering pillar (7)
I believe the answer is:
obelisk
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">four-sided tapering pillar</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 obelisk that I've seen before include "Tapering square pillar" , "Tapering pillar erected as monument" , "Cleopatra's Needle, for instance" , "Pyramidal monument" , "Monument with four sides and tapering top" .)