Rung of a ladder (4)
I believe the answer is:
step
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">rung of a ladder</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for step that I've seen before include "Part of a stairway" , "one stage of flight" , "Stride" , "Footfall" , "Football" .)