Suddenly everyone's in agreement about leader of council (3,2,4)
I believe the answer is:
all at once
'suddenly' is the definition.
(thesaurus)
'everyone's in agreement about leader of council' is the wordplay.
'everyone' becomes 'all' (I've seen this before).
'in agreement' becomes 'atone' (I've seen this in other clues).
'about' is an insertion indicator.
'leader of' says to take the initial letters.
The first letter of 'council' is 'c'.
'atone' going around 'c' is 'atonce'.
'all'+'atonce'='ALL AT ONCE'
(Other definitions for all at once that I've seen before include "Suddenly" , "Without warning" , "How daffodils appeared to Wordsworth" , "in unison" , "Together" .)