Again it's too much (4)
I believe the answer is:
over
I believe 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.
(I've seen this in other clues)
'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" .)