Part of a staircase (5)
I believe the answer is:
riser
'part of a staircase' is the definition.
(I know that riser is part of stair)
This is all the clue.
(Other definitions for riser that I've seen before include "One on the way up" , "Vertical piece between treads of staircase" , "I get up" , "One getting up" , "Upright piece between steps" .)