Vertical part of a stair (5)
I believe the answer is:
riser
'vertical part of a stair' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for riser that I've seen before include "an essential to flight" , "Vertical piece between treads of staircase" , "Vertical upward pipe" , "Vertical pipe; staircase part" , "One who has stopped sleeping" .)