Suddenly, everyone agreed about carbon (3,2,4)
I believe the answer is:
all at once
'suddenly' is the definition.
(I've seen this before)
'everyone agreed about carbon' is the wordplay.
'everyone' becomes 'all' (all people).
'agreed' becomes 'at one' (to be 'at one' is to be agreed).
'about' is an insertion indicator.
'carbon' becomes 'c' (chemical symbol).
'atone' going around 'c' is 'atonce'.
'all'+'atonce'='ALL AT ONCE'
(Other definitions for all at once that I've seen before include "How daffodils appeared to Wordsworth" , "Without warning" , "Suddenly" , "Together" , "Simultaneously - immediately" .)