Part of a stairway (5)
I believe the answer is:
riser
'part of a stairway' is the definition.
('riser' is part of 'stairway')
This is all the clue.
(Other definitions for riser that I've seen before include "Upwardly mobile type" , "Pipe for upward flow" , "Upright piece in stairs" , "upright portion of step" , "Vertical piece between two steps of stairs" .)