Done too much (4)
I believe the answer is:
over
I believe this clue is a double definition.
'done' is the first definition.
(I've seen this in other clues)
'too much' is the second definition.
(I've seen this in other clues)
(Other definitions for over that I've seen before include "Finished -- as a remainder" , "Action far more drastic than necessary" , "Finished - across - in excess" , "Finished like round of bowling in cricket" , "bridge?" .)