Ruler's length (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">ruler's length</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for foot that I've seen before include "Imperial distance" , "An imperial length or body part" , "Measure - part of body" , "Twelve inches" , "12 inches" .)