Part of a step (5)
I believe the answer is:
riser
'part of a step' is the definition.
('riser' is part of 'step')
This is the entire clue.
(Other definitions for riser that I've seen before include "Vertical piece between stair steps" , "Vertical piece between treads of staircase" , "Vertical upward pipe" , "Upright part of stair step" , "Upright piece in stairs" .)