Hermes lacks odd lengths for printer (3)

I believe the answer is:

ems

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'hermes lacks' is the definition.
Although both the answer and definition are plural nouns, I don't see how one could define the other.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for ems that I've seen before include "German river" , "German spa; print measures" , "Units used in printing" , "Printer's units, each two ens" , "spaces at the printing house" .)

I've seen this clue in the Sydney Morning Herald.

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