Tiny child with a large sum (5)
I believe the answer is:
total
'sum' is the definition.
(synonyms)
'tiny child with a large' is the wordplay.
'tiny child' becomes 'tot' (I've seen this before).
'with' means one lot of letters go next to another.
'a large' becomes 'al' (I've seen this in other clues).
'tot'+'al'='TOTAL'
(Other definitions for total that I've seen before include "Ruin" , "given by 8 [ADDING MACHINES]" , "gross" , "French oil and gas company" , "Final tally" .)