Work too hard (6)
I believe the answer is:
overdo
'work too hard' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for overdo that I've seen before include "Exaggerate, go too far" , "Exaggerate; use too much of" , "Take too far" , "Cook wrongly" , "Use too much of (something)" .)