Work too hard in connection with duty (7)
I believe the answer is:
overtax
'work too hard' is the definition.
(to overtax someone is to work them too hard)
'connection with duty' is the wordplay.
'connection with' becomes 'over' (I can't explain this - if you can you should believe this answer much more).
'duty' becomes 'tax' (I've seen this before).
'over'+'tax'='OVERTAX'
'in' is the link.
Can you help me to learn more?
(Other definitions for overtax that I've seen before include "Put too great a strain on, as Revenue Commissioners do" , "Press too hard" , "Require too much" , "Put too much pressure on, like Revenue Commissioners" , "Make excessive demands on (a person's abilities)" .)