Standard of measurement (9)
I believe the answer is:
yardstick
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">standard of measurement</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for yardstick that I've seen before include "Guideline" , "measure of performance" , "Standard base for comparison" , "General measure for comparison" , "Gauge" .)