Too much (6)
'too much' is the definition.
(excess of something means too much of it)
This is all the clue.
(Other definitions for excess that I've seen before include "Glut; surplus" , "Immoderation as in eating, too much" , "Nothing in it, for the Greeks" , "Surfeit; overindulgence" , "debauchery" .)
(Other definitions for excessively that I've seen before include "Overly" , "Overmuch" .)