Child with a large sum (5)
I believe the answer is:
total
'sum' is the definition.
(I know that sum can be written as total)
'child with a large' is the wordplay.
'child' becomes 'tot' (term for a young child).
'with' is a charade indicator (letters next to each other).
'large' becomes 'L' (eg in clothes sizes).
'tot'+'a'+'l'='TOTAL'
(Other definitions for total that I've seen before include "energy provider" , "Complete, absolute" , "French oil and gas company" , "final answer" , "Entire sum" .)