Suddenly a Lancelot with a difference (3,2,4)
I believe the answer is:
all at once
'suddenly' is the definition.
(I've seen this before)
'a lancelot with a difference' is the wordplay.
'with' says to put letters next to each other.
'difference' indicates an anagram.
'lancelot'+'a'='lancelota'
'lancelota' anagrammed gives 'ALL AT ONCE'.
(Other definitions for all at once that I've seen before include "How daffodils appeared to Wordsworth" , "Without warning" , "in unison" , "Together" , "Suddenly" .)