Sum for child, a beginner (5)
I believe the answer is:
total
'sum' is the definition.
('total' can be a synonym of 'sum')
'child a beginner' is the wordplay.
'child' becomes 'tot' (I've seen this before).
'beginner' becomes 'l'.
'tot'+'a'+'l'='TOTAL'
'for' acts as a link.
(Other definitions for total that I've seen before include "Whole, complete" , "The whole, sum" , "Complete; add up" , "The whole number" , "energy provider" .)