Suddenly agreed to knock down dive (2,3,4,5)
I believe the answer is:
at one fell swoop
'suddenly' is the definition.
'agreed to knock down dive' is the wordplay.
'agreed' becomes 'atone' (to be 'at one' is to be agreed).
'to knock down' becomes 'fell' (I've seen this before).
'dive' becomes 'swoop'.
'atone'+'fell'+'swoop'='AT ONE FELL SWOOP'
(Other definitions for at one fell swoop that I've seen before include "all together" , "46 [AT THE SAME TIME]?" .)