Miller is one who keeps students hard at work (7)
I believe the answer is:
grinder
'miller is one who keeps students' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'hard at work' is the wordplay.
'hard' becomes 'grind' (I've seen this before).
'at work' becomes 'er' (I can't explain this - if you can you should believe this answer much more).
'grind'+'er'='GRINDER'
Can you help me to learn more?
(Other definitions for grinder that I've seen before include "Operator of 12 across" , "organist?" , "Salt/pepper mill" , "Eg, pepper mill" , "Peppermill" .)