Race distance (4)
I believe the answer is:
mile
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">race distance</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 mile that I've seen before include "Unit of distance" , "'1,760 yards (4)'" , "Unit of length" , "5280 feet" , "Stretch of road" .)