Whole form replaced in ceremony by child (8)
I believe the answer is:
totality
'whole' is the definition.
(totality is a kind of whole)
'form replaced in ceremony by child' is the wordplay.
I cannot quite understand how this works, but
'child' could be 'tot' (term for a young child) and 'tot' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for totality that I've seen before include "Whole amount" , "The whole of something" , "The complete number or amount" , "Sum" , "Whole thing" .)