Keep down work on printing machine (7)
I believe the answer is:
oppress
'keep down' is the definition.
(I've seen this in another clue)
'work on printing machine' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'printing machine' becomes 'press' (I've seen this before).
'op'+'press'='OPPRESS'
(Other definitions for oppress that I've seen before include "Crush, subjugate" , "Govern harshly, weigh down with care" , "Abuse" , "Keep in subjection and hardship" , "Treat harshly" .)