Printer's measures - they're back inside (3)
I believe the answer is:
ems
'printer's measures' is the definition.
(I've seen this in another clue)
'they're back inside' is the wordplay.
'back' says to take the final letters (the letter at the back).
'inside' becomes 'ms' (I can't explain this - if you can you should give a lot more credence to this answer).
The last letter of 'theyre' is 'e'.
'e'+'ms'='EMS'
Can you help me to learn more?
(Other definitions for ems that I've seen before include "spaces at the printing house" , "Units of print measure" , "German spa; print measures" , "Measurement units in printing" , "German river" .)