Galahad's heart melted Lancelot suddenly (3,2,4)
I believe the answer is:
all at once
'suddenly' is the definition.
(I've seen this before)
'galahad's heart melted lancelot' is the wordplay.
'heart' says to take the centre.
'melted' is an anagram indicator.
The middle letter of 'galahad' is 'a'.
'lancelot' is an anagram of 'llatonce'.
'a'+'llatonce'='ALL AT ONCE'
(Other definitions for all at once that I've seen before include "Together" , "in unison" , "Simultaneously - immediately" , "Suddenly" , "How daffodils appeared to Wordsworth" .)