Youngster with a large sum (5)
I believe the answer is:
total
'sum' is the definition.
('total' can be a synonym of 'sum')
'youngster with a large' is the wordplay.
'youngster' becomes 'tot' (tot is a kind of youngster**).
'with' means one lot of letters go next to another.
'large' becomes 'l'.
'tot'+'a'+'l'='TOTAL'
(Other definitions for total that I've seen before include "French oil and gas company" , "final answer" , "Sum, aggregation" , "Unqualified" , "Complete, entire amount" .)