Suddenly, everyone in agreement about topic, finally (3,2,4)
I believe the answer is:
all at once
'suddenly' is the definition.
(thesaurus)
'everyone in agreement about topic finally' is the wordplay.
'everyone' becomes 'all' (I've seen this before).
'in agreement' becomes 'atone' (I've seen this in other clues).
'about' indicates putting letters inside.
'finally' indicates one should take the final letters.
The final letter of 'topic' is 'c'.
'atone' enclosing 'c' is 'atonce'.
'all'+'atonce'='ALL AT ONCE'
(Other definitions for all at once that I've seen before include "in unison" , "How daffodils appeared to Wordsworth" , "Suddenly" , "Simultaneously - immediately" , "Without warning" .)