Printer's extremely testy stare maintaining order (10)
I believe the answer is:
typewriter
'printer's' is the definition.
(typewriter is a kind of printer)
'extremely testy stare maintaining order' is the wordplay.
I cannot quite see how this works, but
'order' could be 'writ' (I've seen this before) and 'writ' is found within the answer.
'stare' could be 'peer' (I've seen this in another clue) and 'peer' is present in the leftover letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for typewriter that I've seen before include "Former secretarial tool" , "Longest word obtainable using only the top line of a normal keyboard" , "printing machine" , "Hand-operated printer" , "Machine superseded by the word processor" .)