Again it's too much (4)
I believe the answer is:
over
This clue is a double definition.
'again' is the first definition.
(as in the phrases 'start over' and 'start again')
'too much' is the second definition.
(over can mean to too great an extent)
'it's' acts as a link.
(Other definitions for over that I've seen before include "Finished - across - in excess" , "Bout of six bowls in cricket" , "Passed" , "Finished -- as a remainder" , "completed" .)