Area Lancelot abandoned without warning (3,2,4)
I believe the answer is:
all at once
'without warning' is the definition.
(both can mean suddenly)
'area lancelot abandoned' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'abandoned' is an anagram indicator (the original order is abandoned).
'lancelot' with letters rearranged gives 'llatonce'.
'a'+'llatonce'='ALL AT ONCE'
(Other definitions for all at once that I've seen before include "Without warning" , "Simultaneously - immediately" , "Together" , "in unison" , "Suddenly" .)