Vertical pipe (5)
I believe the answer is:
riser
'vertical pipe' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for riser that I've seen before include "Upright piece between steps" , "an essential to flight" , "Vertical part of a stair or step" , "Upright part of stair step" , "Front of a step; vertical pipe" .)