Part of a step (5)
I believe the answer is:
riser
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">part of a step</span>' is the definition.<br/ ><span class="explanation-format__explanation">('riser' is part of 'step')</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)