Printers' measures (3)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">printers' measures</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 ems that I've seen before include "Units of print measure" , "Measurement units in printing" , "German city" , "Printer's units, each two ens" , "spaces at the printing house" .)