Four-sided pillar (7)
I believe the answer is:
obelisk
'four-sided pillar' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for obelisk that I've seen before include "Dagger" , "Tapering square pillar" , "Cleopatra's Needle, for instance" , "Cleopatra's Needle, perhaps?" , "Pyramidal monument" .)