Printers' measures (3)
I believe the answer is:
'printers' measures' is the definition.
(I've seen this in another clue)
This is the entire clue.
(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" .)