Too much on top (4)
I believe the answer is:
over
I believe this clue is a double definition.
'too much' is the first definition.
(over can mean to too great an extent)
'on top' is the second definition.
(I've seen this before)
(Other definitions for over that I've seen before include "Bout of six bowls in cricket" , "See 23" , "completed" , "See 12" , "Finished like round of bowling in cricket" .)